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Heat oven to 180C/160C fan/gas 4. Grease and line a deep, 20cm round cake tin. Beat the butter and sugar together until pale and fluffy, then gradually beat in the eggs, one by one. Fold in the flour, baking powder, mashed potato, zests and juices. Scrape into the tin and level the top. Bake for 45 mins until golden and a skewer poked in comes out clean. Cool in the tin for 10 mins, then move to a wire rack.
Sift the icing sugar, then mix in enough of the lemon juice to make a stiff, but still runny icing. Spread over the cake so it dribbles down the sides a little. Add some chocolate eggs, if using. Leave until the icing has set, then slice and serve. Delicious with a dollop of natural yogurt.