i have used this on alot of cakes with brown frosting.
1 can of chocalate frosting ( DH, BC, PILS work good)
2 cups of powdered sugar
Take frosting out of can and put in bowl. Add powdered sugar and beat on med till well mixed. If frosting is to thick, add a lil hot coffee creamer or milk. If frosting is too thin add some more powdered sugar to get the consistency that you need.