Grease an 18cm/7in round cake tin, line the base with greaseproof paper and grease the paper
Cream the butter and sugar together in a bowl until pale and fluffy
Beat in the eggs, one at a time, beating the mixture well between each one and adding a tablespoon of the flour with the last egg to prevent the mixture curdling
Sift the flour and gently fold in, with enough milk to give a mixture that falls slowly from the spoon
Fold in the lemon zest
Spoon the mixture into the prepared tin and lightly level the top
Bake on the middle shelf of the oven for 30-40 minutes, or until golden-brown on top and a skewer inserted into the centre comes out clean
Remove from the oven and set aside to cool in the tin for 10 minutes, then turn it out on to a wire rack and leave to cool completely