Heat the oven to 170°C/150°C fan/gas 3½. Put all the cupcake ingredients in a large mixing bowl. Using an electric hand mixer or wooden spoon, beat together to a thick smooth batter (this will take about 5 minutes).
Line the muffin tin with the paper cases and evenly spoon in the batter. Bake for 20-25 minutes until risen, golden and a skewer inserted into the centre comes out clean. Leave to cool in the tin on wire rack.
Meanwhile, make the buttercream. Put the softened butter in a large mixing bowl and beat using an electric hand mixer or wooden spoon. Add half the icing sugar and beat until combined, then add the remaining sugar with the vanilla. Whisk or beat until smooth and paler in colour.
