Preheat the oven to 180C/160C Fan/Gas 4. Line a 12-hole cupcake tin with 12 paper cases.
In a large bowl, cream together the butter and sugar using an electric mixer until the mixture is pale and fluffy. Beat in the eggs one at a time, mixing until the egg is completely incorporated into the batter before adding the next. Add a tablespoon of flour if the mixture curdles.
Mix in the vanilla extract and fold in the flour using a large metal spoon until no traces of flour are visible. Gently stir in the milk to loosen the mixture.
Spoon the mixture equally between the cases, about 55g/2oz each. Bake for 25 minutes, or until the cakes spring back when the centre is pressed gently with a finger.
Remove from the oven and take the cakes out of the tin after about 5 minutes. Place them on a wire rack to cool completely.
To make the buttercream icing, sift half the icing sugar into a bowl with the softened butter and beat until light and fluffy. Beat in the remaining icing sugar then beat in the vanilla extract and milk.
Using a palette knife or spatula, spread about a tablespoon of icing onto each cake. This buttercream makes enough for a modest swirl if you want to use a piping bag.