Preheat the oven to gas 4, 180°C, fan 160°C. Line a 12-hole cupcake tin with paper cases.
Place butter and sugar in a large mixing bowl and beat with for few minutes until pale and fluffy.
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 spatula.
Divide the batter between the placed 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.Meanwhile make the buttercream icing. Put all the ingredients in a large mixing bowl and beat & whisk for 4-5 minutes until pale and fluffy.
Swirl the icing over the cooled cupcakes to serve. Store in an airtight container for up to 3 days.