I use the box cake mix (don't remember what brand) of Chocolate Fudge, add 1 c. of mayo and an extra egg. I alternate what I frost and fill with depending on the colors I want. For the cake in my profile I frosted with a chocolate buttercream and filled and detailed with the chocolate ganache buttercream.
For my choc BC, I use the Wilton basic BC and add Nestle Tollhouse cocoa. Some people swear by Hersheys, but I LOVE Nestle. Here is the recipe for the Choc Ganache Buttercream. I think I found this on Baking911...Chocolate Fudge Buttercream Ganache Recipe
Generously frosts a two 8- or 9-inch layer cake with enough left over to pipe a top and bottom border.
1 cup heavy cream
1/4 cup corn syrup, light
2 sticks (1 cup) butter
2 bags (12 oz each) Nestle semi-sweet chips
2 cups sifted powdered sugar
2 teaspoons vanilla extract
Heat cream, corn syrup and 1 stick (1/2 cup) butter on the stove until it JUST BARELY comes to a boil.
Place chocolate chips in a medium-sized bowl. Pour cream mixture over chocolate chips. Let stand 1 minute and stir until smooth. Let mixture cool slightly (but not harden) for a few minutes. The ganache should cool to tepid, about 100 degrees F.
In a mixer bowl, beat together 1 stick (1/2 cup) butter and confectioner's sugar until light and smooth. Add in the cooled ganache and vanilla extract and beat until smooth and light.
NOTE: Frost the cake while the buttercream is at room temperature. (If refrigerated, let sit to room temperature before using. If the buttercream gets too stiff to work with, even at room temperature, beat on low with a mixer. Overbeating the mixture can cause it to become gritty. If necessary, beat in a small amount (1 or 2 tablespoons) of heavy cream to soften slightly. However, the more cream you add, though, the softer the end buttercream will be. Stiffen with more powdered sugar.
I then garnished with white chocolate curls and shavings. You're making me hungry now!!!