75 grams butter (room temperature)
75 grams creamcheese (room temperature)
300 grams icing sugar
milk
1/2 teaspoon vanilla extract
natural food colouring (optional)

Whip the butter with an electric beater until it is creamy and pale. Add the cream cheese and continue to beat. Add vanilla and two tablespoons of milk. Now add your icing sugar gradually, continuting to beat. The trick is to reach the right balance of icing sugar and milk to get the perfect consistency for piping. If you have added all your icing sugar and the mixture is too dry, add a little milk. If it is is too wet, add some more icing sugar. You can never specify the perfect measurements for this as the butter/milk/creamcheese will always be a slightly different temperature. You'll know that you have the right consistency when you dip your finger in and pull out a stiff but perfect peak of icing that doesn't collapse.

I tend not to use food colouring as it is hard to get the right colour when you're using butter. I tried to make baby pink for a friend's birthday last year and we ended up with lurid orange. I've learned from my mistake. You can buy butter alternatives from cake decorating shops, but in my experience they taste discusting and are no replacement for butter.