1cup(2 sticks) unsalted buttersoftened to room temperature
3/4cupDutch processed cocoa powder
2 and 1/2teaspoonsvanilla extract
1/3 to 1/2cupmilk or cream
Preheat the oven to 350ºF. Grease and flour (or use homemade cake release) two 8-inch round cake pans. Set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar on medium-high speed until light and fluffy, about 2-3 minutes. Stop the mixer and scrape down the sides and bottom of the bowl. Beat in the eggs, one at a time, until fully incorporated. Beat in vanilla extract.
Reduce the mixer speed to low and slowly add the dry ingredients. Beat until just combined. Add the buttermilk and continue to beat on low until fully incorporated. Stop the mixer, scrape down the sides and bottom of the bowl, then increase the speed to medium and beat for an additional 2-3 minutes.
Divide the batter evenly between the prepared pans. Bake for 23-27 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.
In a medium size bowl with a handheld mixer or in the bowl of stand mixer fitted with the paddle attachment, beat butter on medium speed until smooth (about 3 minutes).
Decrease mixer speed to low. Add powdered sugar, cocoa, vanilla extract, and milk/cream. Increase mixer speed to to high and beat until completely combined (approximately 2-3 minutes).
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 chocolate buttercream frosting. Spread evenly with an offset spatula.
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. 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.
It is important to use room temperature eggs when working with room temperature butter as it incorporates into batter much more evenly.