120g (4.23 ounces) butter
1 1/3 cups icing sugar
1/2 cup cocoa powder
1-2 tsp milk
Leave the butter to soften at room temperature then beat together with the icing sugar and cocoa powder and 1 tsp of milk.
Beat until smooth adding extra tsp of milk if needed.
Note for a more intense choc flavour frosting beat this in equal quantities