I have some leftover plain white BC (Indydebi's recipe)...and want to make it chocolate flavor...and I think I have 2 cups left...what quantity should I add the coco powder in or should i add melted chocolate squares? and what quantity? Any ideas?

I use cocoa. I throw some in until it looks and tastes the way I want it to. You don't have to measure stuff like this. Just start experimenting!

I, like Leahs just keep adding cocoa until I like the taste and color. I'm thinking for 2 cups of white you will need to add about 1/3 cup of cocoa but as leah said, just experiment until you get it right.
Adding that much powder will make the icing a bit dry so you will probably want to add from 1 to 3 teaspoons of water to it.

I also do as posted above. Add cocoa until it looks right!