1/2cup(1 stick) unsalted buttersoftened to room temperature
1/3 to 1/2cupmilk, cream, or water
DARK CHOCOLATE CAKE
Preheat the oven to 350ºF. Grease and flour (or use homemade cake release) two 8-inch round cake pans. Set aside.
In a large bowl, whisk together the granulated sugar, brown sugar, flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
In a small bowl, whisk together the eggs, vegetable oil, and vanilla extract until completely combined. Pour the wet ingredients into the dry ingredients, add the coffee, and whisk or stir the batter until everything is combined.
Divide the batter evenly between the prepared pans. Bake for 28-30 minutes, or until a toothpick inserted into the center of the cake comes out mostly clean. Remove from the oven and allow to cool completely on a wire cooling rack before assembling.
PEANUT BUTTER FROSTING
In a medium size bowl with a handheld mixer or in the bowl of stand mixer fitted with the paddle attachment, beat butter and peanut butter on medium speed until smooth (about 3 minutes).
Decrease mixer speed to low. Add powdered sugar, vanilla extract, and milk/cream/water. Increase mixer speed to to high and beat until completely combined (approximately 2-3 minutes). If frosting is too thick, add more liquid to desired consistency.
ASSEMBLE THE CAKE
Trim cooled cake layers to create a flat surface. You can do this with a large serrated knife or a cake leveler. Place one layer on a plate or cake stand and cover the top with peanut butter frosting. Spread evenly with an offset spatula. Sprinkle with chopped peanut butter cups, if desired.
Place second layer on top, cut surface down. For a sturdier cake, refrigerate at this point for about 10 minutes. When cake is slightly chilled, remove from refrigerator and spread more frosting on the top and around the sides. Decorate with more chopped peanut butter cups, if desired. Serve. Leftovers stay fresh, covered tightly, at room temperature up to 5 days and in the refrigerator up to 1 week.
Make ahead: prepare cakes and frosting up to 1 day in advance. Cover cakes tightly and keep at room temperature or in the refrigerator until ready to assemble. Refrigerate prepared frosting in an airtight container until ready to use. Unfrosted cake layers may be frozen, wrapped tightly, up to 2 months. Thaw overnight in the refrigerator before assembling. Frosted cake can be frozen, wrapped tightly, up to 2 months. Thaw overnight in the refrigerator and bring to room temperature before serving.
*This means brew a cup of coffee then add water until the level reaches 1 and 1/4 cup total.Adapted from King Arthur