Place the butter in the bowl 
of an electric mixer and beat 
on medium, using paddle attachment, until pale and fluffy. This will take a few minutes.

Add ½ the icing sugar and beat on low until combined.

Add the milk and remaining sugar, then beat on low 
until combined.

Increase 
speed to high and beat until pale and fluffy.

If it is a little too thick, add ½ a tablespoon of milk, mix and then re-assess texture.

If it is too runny, add a little extra icing sugar.

Add colouring if using and mix until thoroughly distributed.