Sift the icing sugar into a large bowl to remove any lumps and a small pinch of salt. Cream the butter and sugar using an electric mixer until it almost forms a paste. Add the vanilla flavouring, followed by the milk. Continue beating until the mixture is smooth and glossy and has a good piping or spreading consistency.
If you find the consistency of the icing is not quite right you can fix it by either adding more sugar if your icing is too liquidy or more milk if the icing is too thick.
Once you've mastered vanilla icing, you can start experimenting with other flavours. Try adding some maple syrup instead of vanilla essence or some lemon, rose or almond extract for a summer twist.
For best results, leave the butter out so that it comes to room temperature before starting. This makes the process a lot quicker and also gives you much smoother icing.
If you are using a piping bag to pipe your icing, then I recommend doubling up the recipe as you wll definately use more icing. Any left over icing can be put in an airtight container and frozen for later use. Make sure your icing is at room temperature before attempting to pipe it or spread it on to your cupcakes.
By the way, you can freeze ready made and iced cupcakes. These will freeze really well and you can keep a stash of these ready for any cupcake emergency. This is really useful if you suddenly realise there is a party or someone with a birthday at school and you want to make sure you have a treat on hand for your gluten free boy or girl.