Put the chocolate in a heatproof bowl, suspended over a pan of barely simmering water. Stir every now and then until melted.
(Alternatively, melt in the microwave, stirring the chocolate every 20 secs so it doesn’t burn.)
Remove the bowl from the heat and leave to cool a little. Meanwhile, put the butter and icing sugar in another large bowl and mash with a spatula to combine (this will prevent you covering the work surface in icing sugar), then whisk with a hand mixer until smooth.
Sift in the cocoa and pour in the melted chocolate and milk, then mix again until smooth, and use to ice your chosen cake immediately. Will keep in the fridge for up to a week, but remove an hour before using to bring it back to room temperature, and beat well.