1. Preheat oven to 350 degrees Fahrenheit. Grease a nine x 13-inch baking pan and set aside.
2. In the bowl of an electric mixer, beat egg whites until stiff peaks form. Add granulated and vanilla sugars and mix to incorporate. Transfer to another bowl and set aside.
3. In the same electric mixer bowl, combine egg yolks with potato starch, oil, lemon juice, and baking powder. Fold in whites by hand.
4. Pour batter into prepared pan and bake 50 to 60 minutes. (Leave oven on for the nut garnish below.)
5. When cake is cool, cut into one-inch cubes.