I also use my buttercream recipe and melt dark choc, maybe 3 oz and add at the end. Works great. You could use milk choc if you like.

My quick recipe:
1 lb powered sugar
1stick margarine, softened
1/4 - 1/2 cup cocoa
Dash of salt
1 t vanilla
Milk tor spreading consistency
Mix and enjoy. Freezes well.

I make the usual recipe but I beat the butter for about 1 1/2 mins before adding the icing sugar, vanilla and cocoa. Once I add the milk and get the right consistency, I beat it again for at least 3 mins. All this extra beating makes the frosting very fluffy and not so sweet. I've been making my frosting like this since I saw it on Martha Stewart.