Make this from the white buttercream recipe
For every pound of white buttercream - start with a 1/2 c. Hershey's brand cocoa - or another good brand - and stir into frosting. Add 1/4 t. more salt. The frosting will thicken with a addition of cocoa- add milk 1 T. at a time to restore consistency. Make at least 2 hours ahead for the best results. The cocoa will absorb and you may have to add more cocoa and milk to make more chocolatey.