Put the butter, sugar and lemon zest into a large bowl, and beat together using an electric mixer for 5 minutes until light and fluffy. Beat in the eggs, one at a time, followed by the vanilla extract. Mix in the flour until a smooth batter is formed. Scoop the batter into the prepared muffin holes and bake for 15-18 minutes or until golden brown and the cakes spring back to a light touch. Check they are loose in the tin, then carefully remove onto a wire rack and cool.