Put the chocolate in a heatproof bowl over a pan of barely simmering water (make sure the bowl isn't touching the water). Stir until melted. Alternatively, melt in the microwave, stirring every 30 secs. Leave the melted chocolate to cool for 5 mins.
Mash the butter and icing sugar together with a fork first, then switch to a wooden spoon or electric beaters if you have them. Sift in the cocoa and pour in the melted chocolate, a pinch of salt and the milk, then mix again until smooth. Use to top cupcakes or cover and fill a chocolate sponge cake.