In a large bowl mix the sugar and orange zest together by hand until the sugar is wet and fragrant. Add in the butter and whisk until pale yellow.
Measure out the dry ingredients in another bowl - the almond meal, polenta, salt and baking powder. Keep aside.
Separate the egg yolks and the whites. Whisk the whites until stiff. Add a pinch of salt to it. Keep aside.
To the butter mix add the egg yolks, lemon juice and orange juice. Whisk well. Add in all the dry ingredients and whisk until just mixed.
Cut and fold the egg whites into this cake until most of the egg is mixed in without losing air.
Line a baking tin with butter paper and pour the cake into it. Bake for 35 minutes at 185 degree centigrade.
Once the cake is baked, remove and cool. Make the lemon icing by mixing the lemon juice with icing sugar. Pour over the cake. Grate some zest on top and serve