In a bowl, beat the butter and sugar until pale and smooth. Crack the eggs into a small jug and beat. Slowly add the eggs to the sugar and butter mixture, beating continuously making sure to incorporate as much air as you can.Add the zest, then sift in the flour and baking powder. Fold in to combine. Gently stir in 2½ tablespoons of lemon juice and the milk. Divide the batter evenly between the moulds.