Preheat the oven to 180°C, fan 160°C, gas 4. Put the paper cake cases in a 12-hole muffin tin.
Put the butter, sugar and vanilla extract in a bowl and beat together until pale and fluffy.
Crack the eggs into a separate bowl and whisk. Add a little egg to the butter and sugar mixture and beat, and then continue to add the egg a little at a time. Add 2 spoonfuls of the flour – this will help stop the mixture from curdling.
When you have added all the egg, sift in the remaining flour and the baking powder.
Fold together using a large spoon until all the flour is mixed through. Don’t beat it or you will knock out all the air.
Spoon the mixture into the paper cases. Bake for about 12-15 mins, until the cakes are risen, golden and feel slightly springy when you touch them. Leave to cool in the muffin trays.
To decorate, mix just enough water into the icing sugar to make a thick icing then spread over the top of the cakes. Decorate with sprinkles and other decorations you like.