2 tablespoons butter or margarine
2 tablespoons cocoa
1/2 teaspoon vanilla extract
2 tablespoons water
1 cup confectioners' (powdered) sugar
In a small saucepan, melt butter over low heat. Add cocoa and water, stirring constantly until mixture thickens. Do not boil. Remove from heat and cool slightly. Blend in confectioner's sugar and vanilla. Spread over cake.
For a thicker icing-type topping, cool butter/cocoa mixture completely before adding sugar and vanilla.