First, make the cream filling since it needs to be completely cooled. Heat the milk with the lemon zest. In a separate bowl, mix the egg yolks, sugar, remaining grated lemon zest (we love lemon!), and potato starch.

Pour this mixture into the warmed milk. Stir and cook over medium heat, stirring constantly, until the mixture becomes thick and creamy.

For the cake batter, beat the eggs with the sugar, grated lemon zest, juice of half a lemon, milk, and oil in a bowl. Add the flour and vanilla baking powder and mix until smooth.