Put butter and chocolate into a saucepan

Heat gently, stirring until butter and chocolate have melted

Add 1/2 cup of the icing sugar

Stir until mixture is thick enough to handle

Stir in rum and cocoa

Add enough of the remaining icing sugar to make a stiff mixture

Measure tablespoonsful of the mixture and shape into balls

Roll in coconut

Chill until firm.