Mix together the butter and sugar until very light and smooth. Combine flour and baking powder, then add to the butter mixture alternately with the eggs. (The longer you mix the more air gets into the mix making them light and fluffy at the end result.) Finally, add the vanilla. Divide cupcake mixture between 12 cupcake cases.
Bake in a preheated 160 C / Gas 3 oven for 25 to 30 minutes, or till a skewer inserted in the centre comes out clean. Be sure not to open the oven door while they're rising!