3 cups confectioners' sugar
3/4 cup unsweetened cocoa
1 stick butter, softened
3 - 4 tablespoons heavy cream
1 teaspoon vanilla

Sift sugar and cocoa together into a large bowl. In another bowl combine 1 cup of sugar mixture with butter and 1 tablespoon cream, beating until smooth. Add another cup of sugar and another tablespoon of cream, beating well. Continue until all ingredients are combined and frosting is fluffy. Beat in vanilla.

From Food Network.com