Place the sugar, softened butter, lemon zest and the seeds of a small vanilla bean in a large mixing bowl.
Mix on medium speed until light and fluffy.
Place the 3 egg yolks and 2 whole eggs into a measuring cup. This makes it easier to mix in the eggs a little at a time.
With the mixer on medium speed, pour in the eggs a little at a time and mix well until fully incorporated. If using a stand mixer, stop and scrap the bowl down with a rubber spatula.
Sift in the flour, potato starch and baking powder.
Mix until the flour mixture is well incorporated.
In a separate medium-sized bowl, beat the reserved egg whites while slowly mixing in 1/3 cup of powdered sugar until the peaks hold their shape.
Use a rubber spatula to fold in the beaten egg whites until no visible egg whites are remaining.
Pour the cake batter into a greased and floured cake pan.
Bake in a preheated oven set at 350 degrees F, for about 35 minutes or when the top springs back when lightly touched. You can also test for doneness with a toothpick. When a toothpick inserted comes out clean then it's done.
Take care not to overbake the cake.
I let the cake cool in the pan before releasing it because it's very soft. Run an offset spatula around the edges before releasing the cake from the baking pan.
Once cooled dust with powdered sugar and serve.
Alternately, you can serve individual slices with a dollop of sweetened whipped cream, drizzled with honey and topped with a strawberry.
You can also style it for a show-stopping dessert. See the notes below on how to style it.
Just so you know, the Nutrition Information listed below does not include the sweetened whipped cream and strawberries, nor the honey as they are optional when making this recipe.