In the microwave or on in a saucepan, melt together ½ cup of the butter (1 stick) cocoa and milk and stir until smooth. Cool slightly.
In another large bowl, beat remaining butter and vanilla together, then add cooled cocoa mixture.
Add confectioners' sugar 1 cup at a time, blending with each addition, until frosting is smooth and creamy. For a stiffer frosting for piping, use all 8 cups of sugar. For a softer frosting for swooping on a cake, use just 7 cups of confectioners' sugar.