1. Beat egg yolks with two tablespoons sugar and vanilla sugar. Add potato starch, oil, zest, and juice.
2. Whip egg whites and remaining sugar to stiff peaks. Gently fold yolk mixture into whites.
3. Pour into prepared pan and bake 15–18 minutes. Do not over-bake.
4. Preheat oven to 350 degrees Fahrenheit. Line an 11- by 17-inch pan.