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.