Preheat oven to 350ºF.
Whisk eggs until frothy, then slowly add sugar, butter and lemon and orange rinds, beating thoroughly after each addition.
Fold in flour and baking powder.
Grease an 8-inch cake pan, pour in the mixture.
Bake for one hour or until the cake is golden brown and well risen; a toothpick inserted into the center should come out clean.
When cool, dust with powered sugar.