Line a 12-hole muffin tin with paper cases and heat the oven to 200C/180C fan/gas 6. Beat the butter and sugar together in a stand mixer, or use an electric whisk. Slowly whisk in the egg, a little at a time, along with a spoonful of the flour and the vanilla extract. Fold in the remaining flour, the baking powder and sprinkles. Divide the batter between the cases and bake for 18-20 mins until lightly golden and a skewer inserted into their centres comes out clean. Leave to cool for 5 mins in the tin, then transfer to a wire rack to cool fully.

For the buttercream, beat the butter using an electric whisk or in a stand mixer until softened slightly, then beat in the icing sugar in three additions until fully incorporated. Add a splash of water to loosen, then the vanilla extract. Spread the buttercream over the cooled cupcakes or spoon into a piping bag and pipe swirls on top. Decorate with more sprinkles. Will keep in an airtight container for three days.