1: Preheat the oven to gas 4, 180°C, fan 160°C. Line a 12-hole cupcake tin with paper cases. Put the butter and sugar in a large mixing bowl and beat with an electric whisk (or by hand with a wooden spoon) for 2-3 mins until pale and fluffy.
2: Add the eggs one at a time, beating well after each addition. Add the vanilla extract and beat briefly to combine. Gently fold in the flour and just enough milk to make a smooth batter that drops easily off the spoon.
3: Divide the batter between the cases, then bake for 20-25 mins or until a skewer inserted into the middle comes out clean. Transfer the cakes to a wire rack and leave to cool completely.
4: Meanwhile, make the buttercream icing. Put all the ingredients in a large mixing bowl and beat with an electric whisk for 4-5 mins until pale and fluffy.
5: Swirl the icing over the cooled cupcakes to serve. Store in an airtight container for up to 3 days.