Start by preheating the oven to 180°C
Separate the whites from the yolks.
Add the white sugar and vanilla sugar to the yolks, and whisk.
Zest the lemon and add it to the mix.
Pour in the flour and cornstarch and mix.
Whisk the egg whites until they are stiff, then add them little by little to the first mixture using a rubber spatula, mixing gently.
Pour one half into a 15cm silicone mould.
Bake for 30 minutes at 180°C.
Remove from the mould and cool on a wire rack.
Pour the second half of the dough into the mould.
Bake again for 30 minutes at 180°C.