Heat oven to 180C/160C fan/gas 4. Line 12 holes in a shallow muffin tin with cupcake cases (not muffin cases). Cream the butter and sugar together until the mixture is light and fluffy. Beat the eggs and vanilla in a jug, then beat into the butter mixture a little at a time.
Fold the flour into the mixture along with a little milk – the mixture should fall easily off a spoon. Divide the mixture between the cases, filling each one no more than half full.
Cook the cakes for 12-15 mins – they should be firm to the touch and slightly golden brown. Cool in the tin for 5 mins, then lift them out gently and leave to cool completely.
To make the icing, beat the butter until it is very soft. Beat in the icing sugar a little at a time, then beat in a splash of the milk as you need to give you a soft icing that can be piped easily. Scrape the icing into an icing bag fitted with a star nozzle. Ice the cakes in swirls, starting in the centre and working outwards.