Beat the egg whites with granulated sugar in a thick foam. In another bowl whisk the soft butter with powdered sugar cream. Under continuous stirring, add the egg yolks (one by one). Mix the flour, starch, baking powder and salt. Add flour mixture in portions to the egg and mix. Finally, add the lemon zest and egg whites and gently mix. Pour the mixture into a baking dish (covered with baking paper) and bake at 180 C about 30-35 minutes. The finished cake sprinkle with powdered sugar and serve. You can cut the cake bases and make them blind cream optional.