Place the butter in the bowl of an electric mixer and beat until softened.
Add 2 cups of the icing sugar to the bowl and beat just until they are incorporated then add the rest of the icing sugar. Beat until it is mixed in then increase the speed and mix until it is light and fluffy (a few minutes).
Add the cream and beat it through then add the food colouring a little at a time until you reach the desired colour.
Grate the chocolate into the bowl and gently fold it through. Don't mix it too much or the chocolate will melt into the icing.