175g icing sugar
2 tbsp milk
100g melted chocolate
a pinch of salt
Melt the chocolate and put to one side to cool.
Using a freestanding mixer or a handheld mixer, beat the butter until pale and fluffy, this should take around 5 minutes.
Sieve the icing sugar in to the butter in two parts, beating well after each addition.
Check your chocolate is not too warm before adding to the butter mixture with the salt, beat again until everything is well combined.
Finally add the milk and beat to loosen the mixture (you may not need all of the milk).