Preheat the oven to 170 °C (325 °F), on the conventional setting.
Separate the egg yolks from the whites and set the whites aside.
Add the sugar, ½ teaspoon of natural vanilla extract, and the lemon zest to the egg yolks, then beat with an electric whisk for 3 minutes or until the mixture is light and fluffy.
Add the potato starch all at once, then mix with an electric whisk for a few minutes.
Using a whisk, stir the milk into the egg mixture and set aside.
Beat the egg whites to stiff peaks with an electric mixer, then fold them into the milk mixture a little at a time with a spatula.
Line the pan with wet and squeezed parchment paper. If you're not using parchment paper, thoroughly butter the bottom and sides of the pan.
Pour the mixture into the pan and prepare an aluminum foil cover; place it over the pan halfway through baking to prevent the surface from becoming too brown.
Bake in a preheated oven for 50 minutes, or until the center is firm (test with a toothpick).
Remove from the oven, let cool, and chill in the refrigerator for a few hours or overnight.
Remove the cake from the pan. Then hold the cake with one hand while removing the parchment paper, place it on a serving plate, dust it with powdered sugar, and serve it cut into diamond shapes or squares.