Making a cake and have tons of white buttercream. How would I go about making it chocolate? Do I just add some baking cocoa?? Any help would be greatly appreciated!!

Beat cooled melted dark chocolate into it.

ETA: I do use cocoa sometimes (or both), but the cocoa dissolves best if you add it in with the first of the powdered sugar.

I do what Coral3 does as well. I mix in cocoa and then melted chocolate as well. I don't measure it out, just keep adding it until it gets as chocolately as you want!

The easiest way is to mix "Nestle Choco Bake" in your icing while beating. Choco bakes are premelted(single use packages) chocolate flavor. You can find them in any supermarket baking isle.
HTH

I add chocolate ganache to it.

Thanks guys! I will give a few of these ideas a try.