Fudgy, beyond rich chocolate sheet cake topped with the creamiest peanut butter frosting. Feeds a crowd! I encourage you to read the full recipe and the recipe notes before beginning so you’re completely prepared.
- 1 cup (230g) unsalted butter
- 1 cup (240ml) water
- 1/2 cup (41g) unsweetened natural cocoa powder (not dutch-process)
- 2 cups (250g) all-purpose flour (spoon & leveled)
- 1 teaspoon baking soda
- 1/4 teaspoon salt
- 2 large eggs, at room temperature
- 1 cup (200g) granulated sugar
- 1/2 cup (100g) packed light brown sugar
- 2 teaspoons pure vanilla extract
- 1/2 cup (120ml) buttermilk, at room temperature*
Peanut Butter Frosting
- 1 and 1/2 cups (405g) creamy peanut butter (do not use natural style)
- 1/2 cup (115g) unsalted butter, softened to room temperature
- 3 cups (360g) confectioners’ sugar
- 1/2 cup (120ml) heavy cream, half-and-half, or whole milk*
- 1 teaspoon pure vanilla extract
- pinch salt
- optional: 1/2 cup (135g) melted creamy peanut butter and 1 cup chopped peanut butter cups
- Preheat oven to 350°F (177°C). Generously grease a 12×17 inch half sheet/jelly roll pan. Set aside.
- Heat the butter, water, and cocoa powder together in a large saucepan over medium heat. Whisk constantly until the butter has melted and everything is smooth and combined. Set aside. In a large bowl, whisk the flour, baking soda, and salt together. Pour the warm chocolate/butter mixture on top, then slowly whisk together until combined. It will be very thick.
- Whisk the eggs, granulated sugar, and brown sugar together in a medium bowl. The mixture will be thick. Whisk in the vanilla until combined, then pour into the chocolate batter and whisk until combined. Finally, whisk in the buttermilk. Pour the chocolate batter into the sheet pan.
- Bake for 15-20 minutes. (Mine takes exactly 17 minutes.) Use a toothpick to test for doneness: stick it into the cake. If the toothpick comes out clean, the cake is done. If not, continue baking. Remove cake from the oven and set the pan on a wire rack. Allow the cake to cool completely in the pan before frosting. The refrigerator helps speed things up, if you have room for the large pan!
- Make the frosting: Using a handheld or stand mixer fitted with a whisk attachment, beat softened butter and peanut butter together on medium-high speed for 2 minutes until completely smooth and creamy. Add the confectioners’ sugar, cream, vanilla extract, and salt. Beat on low speed for about 20 seconds, then gradually increase to high speed. Beat on high for 1 minute until combined and frosting is fluffy. Taste; add more salt if needed. To thicken, add more confectioners’ sugar and to thin, add more cream.
- Spread frosting all over cooled cake, and then top with toppings, if using. Slice and serve. Cover leftovers tightly and store in the refrigerator for up to 5 days. It’s so good cold!
- Make Ahead & Freezing Instructions: Cake can be made 1 day in advance, covered tightly at room temperature, and then frosted before serving. Frosting can also be made 1 day ahead of time– cover tightly and keep in the refrigerator. Bring frosting to room temperature before spreading. Frosted cake can be frozen up to 2 months; thaw overnight in the refrigerator and bring to room temperature before serving.
- Buttermilk: Buttermilk is required for this recipe. You can make your own DIY version of buttermilk if needed. Add 1 teaspoon of white vinegar or lemon juice to a liquid measuring cup. Then add enough whole milk to the same measuring cup until it reaches 1/2 cup. (In a pinch, lower fat or nondairy milks work for this soured milk, but the cake won’t taste as moist or rich.) Stir it around and let sit for 5 minutes. The homemade “buttermilk” will be somewhat curdled and ready to use in the recipe.
- Heavy Cream: Using heavy cream, half-and-half, or whole milk in the frosting produces the creamiest texture. (I use and highly recommend heavy cream.) If needed, you can use a lower fat or nondairy milk.
- Room Temperature Ingredients: Here is why the ingredients must be at room temperature. The butter is melted and warm, so it’s best the other ingredients are not cold.
- Toppings: We usually top this cake with melted peanut butter and chopped peanut butter cups. Use what you like best such as Reese’s Pieces or chocolate sprinkles. In some of the photos above, I also topped the cake with these Reese’s peanut butter sprinkle ice cream topping, a product I found near ice cream aisle.
- Sheet Pan: I own and love this 12×17 half sheet pan.
- Cake batter adapted from All Recipes.
Keywords: chocolate sheet cake, chocolate cake with peanut butter frosting