What you need...
2 1/2 lbs. (1135 g) Vanilla Buttercream (any buttercream will work)
4 oz (113 g) Black Onyx Cocoa powder
1 Tablespoon black gel food coloring
How to make this....
1.) Take a small amount of buttercream in a small bowl and heat gently in the microwave to melt.
2.) Add cocoa powder to the melted buttercream and stir to dissolve.
3.) Add this mixture, along with the gel food coloring into the remaining buttercream and stir until fully incorporated.