Mix polenta, caster sugar, almonds and baking powder. Beat together eggs, oil and zests and then beat it into the dry mix. Pour into a 20cm greased and lined cake tin (preferably loose base or spring form). Place in a cold oven and turn oven onto 190°C (normal oven, not fan), gas mark 5. Bake for 40 minutes. Test with a thin knife/cake tester. If it comes out clean the cake is ready. The cake may seem a little wobbly still but it will carry on cooking in the tin after it's out of the oven.
Leave in tin until cold and then remove it and ice. For a drizzle icing mix 40g soft butter and 2 teaspoons of milk with 80g icing sugar and 2 large teaspoons of lemon curd. Beat lightly and pour over the cake. A chocolate ganache icing (with added orange rind) works well too.