Preheat the oven to 170°C (fan 150°C, gas mark 3). Line a muffin tray with 10-12 paper cases.
Beat together the butter and sugar, then add in the eggs one at a time mixing between each addition. Add the vanilla extract.
Carefully fold in the flour until just incorporated.
Spoon the mixture into the paper cases and bake for 15-20 minutes until golden on top. Insert the tip of a knife into the cases to check that they are baked throughout. If they are ready the knife should be clean when removed from the sponge.
Allow to cool on a rack.