I've tried to make black & white cookies 4 times and every time the chocolate icing always looses it's glossiness, turns a lighter shade of brown and is powdery looking.
I use 3 cups sifted powdered sugar, 1/4 cup Ghiradelli 100% cocoa powder, 2 tablespoons of light corn syrup and warm water. Combine until smooth with a whisk and do not over mix. It dries glossy at first but within hours they start to turn dull and powdery looking.
A few hours after icing.