Pre-heat the oven to 180°C.
Grease an 18cm round cake tin and line the base with greaseproof paper.
Cream the butter and sugar together in a bowl until pale and fluffy.
Beat in the eggs one at a time, adding a tablespoon of flour with the last egg.
Sift the remaining flour and gently fold it into the mixture.
Add enough milk to achieve a consistency that falls slowly from a spoon.
Fold in the lemon zest and transfer the mixture to the prepared tin.
Bake on the middle shelf for 30-40 minutes until golden-brown.
Cool in the tin for 10 minutes before transferring to a wire rack.
Decorate with candied peel before serving.