Step 1
Preheat oven to 350°F. Using a pastry brush, butter two cake pans completely. Dust pans with cocoa powder and tap out the excess.
Step 2
In a medium bowl, whisk the cocoa powder, flour, baking powder, and salt together. Set aside.
Step 3
Over a double boiler, melt chocolate and butter. Stir until combined. Remove from heat and allow the mixture to cool.
Step 4
In a large bowl, whisk sugar, eggs, and vanilla until well incorporated. Add the chocolate-butter mixture and mix. Add the flour mixture and mix until combined.
Step 5
Evenly divide the batter into the prepared pans. Bake for 20 minutes or until a toothpick inserted into the middle of the cake comes out clean. Then remove the cakes from the oven and place over a cooling rack for 10 minutes. Invert the cakes and allow to cool completely.
Step 6
Meanwhile, prepare the filling. In a medium bowl using an electric mixer, beat butter, cream cheese, and vanilla until light and fluffy. Gradually add the powdered sugar and milk. Beat until smooth.
Step 7
When the layers are completely cool, turn one cake over (printed side facing down) over a serving platter. Spread buttercream over the cake and top with the second layer. Cover the sides with sprinkles if desired.