Preheat the oven to 180 degrees C. Grease a round 18cm- diameter cake tin and line with nonstick baking paper. Beat the butter with the sugar, lemon rind and vanilla until ﬂuffy, then beat in the eggs, one at a time. Sift in the ﬂour and baking powder and fold in with a metal spoon.
Spread the mixture evenly in the prepared tin and scatter the citron peel on the top. Bake for 1–1 1/4 hours, or until lightly browned and a skewer inserted in the centre comes out clean. Sprinkle with the sugar and leave to cool in the tin on a wire rack.