This quantity will generously ice 12 cupcakes. If you want just a single star of icing in the middle you will be able to reduce it.
The proportions are easy to remember 150g butter and 300g icing sugar + flavouring of choice. Unsalted block butter is essential here (but you don’t need the expensive stuff – we use Sainsbury’s basics). It needs to be really soft (not melted) so dice it and leave at room temperature for a few hours. Put into a large mixing bowl and whisk (with an electric whisk for ease) until pale in colour. Whisk in the icing sugar and keep whisking for a good 5 minutes until it is almost white. . Don’t skip this bit! Then whisk in the flavouring. For this amount we use about 2 teaspoons vanilla extract.
To stop the icing sugar flying everywhere, use a wooden spoon to mix it with the butter before using the whisk. Alternatively drape a clean tea-towel over your hand and the whisk.