Addictive Recipes from a Self-Taught Baker

Mini Rainbow M&M Cookies with Peanut Butter Buttercream

What is the only way to make a truly perfect cookie… better?  Add buttercream.  Peanut butter buttercream, to be exact.  My Rainbow M&M Cookies are now MINI (i love cute things!!) and double stuffed, over-filled, and bursting with buttery, creamy peanut butter buttercream.

I have a love for the same basic cookie dough recipe, a recipe based off of the Cooks Illustrated recipe for Chewy Chocolate Chip Cookies. I’ve used the dough recipe for several of my recent cookies because…. it’s a guarantee.  This basic recipe produces thick, chewy, unbelievably soft cookies.

The peanut butter buttercream filling is extremely decadent in today’s cookies.  Like, hand-me-a-TALL-glass-of-milk-decadent.  It’s rich and THICK.  It’s much different than the silky peanut butter frosting I used to frost my brownie cupcakes.  No, this buttercream is comparable to the thick sandwich filling in Oreos and I credit that to the smidge of heavy cream I used in my recipe.  You may use any type of milk or even half-and-half, but I had leftover heavy cream in my fridge, so I figured what-the-heck!

A buttercream’s thickness is directly related to the ratio of liquid vs sugar.

I used very little heavy cream in this buttercream and a decent amount of confectioners sugar, thus producing a stick-to-the-roof-of-my-mouth frosting.

The mini M&Ms and peanut butter buttercream filling pair wonderfully together, just as one would imagine.  Peanut Butter M&Ms are my favorite and I get the same fudgy/PB effect in today’s cookie recipe.

Mini Rainbow M&M Cookie Sandwiches with Peanut Butter Buttercream

Buttercream filled cookie sandwiches should be stored in the refrigerator.



  • 2 cups + 2 Tablespoons all-purpose flour
  • 1/2 teaspoon baking soda
  • 3/4 cup (1.5 sticks) salted butter, softened to room temp
  • 1 cup light brown sugar
  • 1/2 cup granulated sugar
  • 1 egg + 1 egg yolk, at room temperature
  • 2 teaspoon vanilla extract
  • 1 and 1/2 cups mini M&Ms

Peanut Butter Buttercream

  • 6 Tablespoons salted butter, softened to room temperature
  • 6 Tablespoons creamy peanut butter
  • 2 teaspoons vanilla extract
  • 3 cups confectioners’ sugar, sifted (plus extra if needed)
  • 1 Tablespoon cream or milk or half-and-half (plus extra if needed)


  1. Preheat oven to 325 degrees.
  2. Make the cookies. Mix the flour and baking soda in a bowl and set aside. With an electric or stand mixer fitted with a paddle attachment, cream the butter and sugars together on medium speed. Add the egg, egg yolk, and vanilla and mix well. Gradually add the flour mixture and stir until a dough forms. Fold in mini M&Ms.
  3. Chill dough for AT LEAST one hour (I chilled mine for 24 hours).
  4. Drop cookie dough into small uniform sized balls (mine were about 1 tsp of dough each, which are TINY, but bake them any size you'd like) onto ungreased baking sheet or silpat. Bake for 8 minutes or until edges are slightly browned. The centers should appear soft and puffy. Let cool completely on wire rack.
  5. Make the frosting. Cream butter and peanut butter on medium-high speed with a stand mixer fitted with a paddle attachment.  Add vanilla extract and 2 cups confectioners' sugar.  Beat for 2-3 minutes, or until mixture is smooth and thick.  Scrape down the sides of the bowl if needed.  Add milk/cream/half-and-half and beat for another minute.  Add 1 more cup of confectioners' sugar and continue adding until you reached desired consistancy.  I added exactly 3 cups.  Once cookies are completely cooled, stuff cookies generously with peanut butter buttercream filling.

Did you make a recipe?

Tag @sallysbakeblog on Instagram and hashtag it #sallysbakingaddiction.

© Sally’s Baking Addiction. All images & content are copyright protected. Please do not use my images without prior permission. If you want to republish this recipe, please re-write the recipe in your own words, or link back to this post for the recipe.


Here are my original, full-sized, un-stuffed Rainbow M&M Cookies:


  1. Amazing! These are my kind of cookie!

  2. Thanks for linking up and trying my PB buttercream and yes, we share a love for super thick frosting. When I made it as well, it was really thick. I use half and half in all my baking. I don’t buy much milk but I always have half-and-half on hand (for coffee..or baking!) so that’s what I used but really, it’s all about the butter/confectioners sugar ratio as you mentioned and basically getting as much sugar into that butter as it will “hold”. The more you cram in, the thicker it’s going to be…which I love 🙂

  3. There is no doubt that these amazing cookies would last one minute at my place 😀

    Choc Chip Uru

  4. Sandwich cookies are so fun. Love these Sally!

  5. Okay, so wait. Back up a second here.

    Not only are these MINI cookies, which means that I’m required by law to eat at least 4 of them, but they are ALSO full of some of the most delicious sounding peanut butter frosting to date?

    Imma need my fat pants.

    • Hahahaha Karly your comments make me laugh uncontrollably. And yes, mini things = required to pop about 4 or 5 into your mouth. Kinda like your mini corndogs!

  6. whoa, these cookies are super intense! i wonder what they’d be like with reese’s pieces instead of the m&ms….

  7. Yum! Anything with peanut butter and M&M’s in the same recipe is all good with me!!

    I sent you an email about helping in a bake sale I am doing – are you interested?

  8. Sally, these cookies are amazing! Such gorgeous pictures, too! So, so glad I found your site! And I’m totally drooling here over that peanut butter buttercream–heaven!!

  9. Sandwich cookies are the best because you get two cookies for the price of one! 🙂

  10. i love that these are mini and are stuffed with peanut butter buttercream!! i can’t think of a better way to eat these..well, a glass of milk to help me out a little but other than that, this is perfection!

  11. Sally, These just look fabulous! I’ve never had peanut butter buttercream, and now I think I’m going to have to try it. I also slways love a good sandwich cookie; it’s such a good excuse to eat 2 cookies:)

  12. Turning these up a notch with the butter cream. For the win!

  13. These are INSANE! I am in total and utter love! Being a frosting girl through and through – a few of these would satisfy my cravings completely.

  14. Oh Sally. I knew these would be good. Sign me up for a few (read 10)! That frosting looks dreamy!!

  15. You just keep posting better stuff every day:-) Good thing I have on stretchy pants…the other ones won’t fit after I get ahold of that frosting!!! Thick and stick to the roof of my mouth sounds just perfect!!!

    • Hahahaha Jocelyn I think I’ll need my stretchy pants if I eat another one of them. Thankfully, I brought the rest into work and they disappeared!

  16. How fun! I haven’t seen mini m&ms in ages!

  17. Your description of that peanut butter buttercream is making me positively drool!

    • haha Sally you are too funny! And yes, thinking about that buttercream (it’s all gone!) is making me positively drool too…

  18. Okay, you twisted my arm. I’ll run and go get some elastic-waistband pants now. Jeez louise, these look superfantastic. Small and rich and creamy and chewy and pretty much the stuff my dreams are made out of. Yeah, most people dream as if they’re in a rom-com, or on a beach, or beating stuff up as a ninja (if you’re my boyfriend), but I dream of this. Perfect, petite cookies stuffed with the most decadent peanut butter frosting evah. I love you.

  19. Sandwich cookies…..peanut butter buttercream….I’m dying! WHY does it have to be 90 degrees in my kitchen right now? What I wouldn’t give for central air. Maybe I’ll just make this frosting and eat it with a spoon.

  20. I love how colourful your blog is! These look beautiful. Also: I love your blog! So I’ve nominated you for the versatile bloggers award. Check out the post to find out more =]

  21. These are too cute! M&M’s are my absolute fav! Next to chocolate covered pretzels of course! 🙂

  22. Why do they have to be Mini sandwich cookies, I would love a big ole sandwich cookie!! Many others do not share my DEEP appreciation for sweets…

  23. Hi Sally! I wanted to make just the cookies for a party, and was wondering if this recipe can be halved. Thanks!

    • Hi Ally! It’s difficult to halve this recipe with success because of the 1 egg + egg yolk. I don’t recommend it. You can always freeze the leftover cookies (without buttercream) for up to 2 months.

  24. Hi Sally! I just made these for a Chrismukkah party tonight. Instead of rainbow M&Ms, I used red & green ones. I also added some festive Christmas sprinkles to the buttercream frosting. To die for! Thanks for sharing!

    • Love the idea of adding sprinkles to the buttercream Anna! 🙂 Thanks for giving me that tasty little tip. I’ll try it next time.

  25. Can I use chunky,PB instead of creamy?

  26. Made these cookies – SO good, and a big hit!

    One question – the batch I did that was about a tablespoon’s worth of dough came out perfectly, but the tiny ones I tried were way too crispy … how long would you recommend cooking those tiny ones? Any temperature adjustment?

    Your site has become my go-to for all new baking adventures 🙂

    • Hey Hallie! No temperature adjustment – about 7-8 minutes in the oven. I need to be more clear in these instructions – I’ll update it!

  27. These were excellent. I just made them for a Labor Day BBQ and it’s been hard trying to get my husband to stay out of them! They are a little different than the average cookie, pretty & fun for a party. I added these to my personal favorite recipe! I have really been enjoying making items from this blog- Thank you!

  28. Hiiii sally 🙂
    I have a question, I dont have light brown sugar (I dont even think that is available in my country) but I do have brown sugar, is it okay if I use it or do you recommend me to use half brown sugar and half regular sugar? or is it light brown sugar mandatory?

    Thank you ♥

Leave a Reply

Your email address will not be published. Required fields are marked *