Step 1:Melt the dark chocolate with the butter together in a bowl over a pan of simmering water.Step 2:Meanwhile, sieve together the icing sugar and cocoa powder in a separate bowl.Step 3:Once melted, add the icing sugar and cocoa powder gradually to the chocolate mix and beat with an electric mixer.Step 4:Add a touch of milk if a thinner consistency is desired.Step 5:Allow to cool before using as a frosting on a cake.