3 to 4 c. powdered sugar
2 to 3 tbsp. butter
milk (by the tbsp. to get the proper consistency to ice cake)
1/3 c. packed cocoa
1/2 tsp. vanilla (optional)

Sift together powdered sugar with cocoa. Melt butter. Pour butter over sugar/cocoa mixture. Use an electric mixer and add milk by the tablespoon to get the icing of consistency you want. If it's too thin, add a bit more powdered sugar. If it's too thick, add another tablespoon of milk.