This chocolate buttercream is made with real chocolate rather than cocoa powder (not that cocoa powder isn't real chocolate!).
Preheat the oven to 350 degrees F. Butter two 6-inch x 3-inch round cake pans. Line with parchment paper, then butter and flour the pans.
Cut both cakes in half (horizontally) to make 2 layers. In total, you will have 4. Place 1 layer, flat side up, on a flat plate or cake pedestal. With a knife or offset spatula, spread the top with frosting. Place the second layer and do the same. Repeat all steps till the cake is completed frosted between the layers. You will then use an offset spatula and apply frosting along the sides for a crumb coat (thin layer). Place cake in the freezer for 15 minutes. Remove and apply a final layer of frosting. Decorate as desired.
In the bowl of an electric mixer fitted with a paddle attachment, beat the butter on medium-high speed until light yellow and fluffy, about 3 minutes. Add the vanilla and continue beating for 3 minutes. Turn the mixer to low, gradually add the confectioners' sugar, then beat at medium speed, scraping down the bowl as necessary, until smooth and creamy. On low speed, add the chocolate to the butter mixture and whip for 5 - 7 minutes until it is nice and fluffy!
I use 6-inch cake pans that are about 3 to 4 inches deep, hence why the cooking time can be anywhere from 30 - 40 minutes! If you cake pans are less deep you will adjust the time accordingly (anywhere from 20 - 30 minutes).