Preheat the oven to 180°C, fan 160°C, gas 4. Line a 12-hole cupcake tin with paper cases (we love our patterned cake cases).
Using an electric hand mixer, beat the butter and sugar together until light and fluffy. Add the vanilla, eggs and 1 tablespoon of the flour. Beat until combined, then whisk in the remaining flour.
Divide the mixture between the paper cases and bake for 15-20 minutes, until risen. Leave in the tin for a couple of minutes, then transfer the cakes to a wire rack and leave to cool completely.
Meanwhile, make the icing. Beat the butter in a large bowl, then add half the icing sugar and whisk until smooth. Add the remaining icing sugar and continue beating until creamy.
Divide the icing between 3 bowls and colour each differently for a pastel rainbow of cupcakes. A little food colouring goes a long way, so start with a drop or two and mix in well.
Spoon the icing into a piping bag fitted with a large star nozzle. Hold the nozzle end with one hand. Squeeze the top of the bag with your other hand to make the icing flow through. Start by making a circle around the edge of the cakes and work towards the middle, making a spiral. Decorate with sprinkles before serving.
Cook’s tip: if you don’t have a piping bag, simply spread the buttercream on using a butter knife.