Preheat oven to 150°C (static heat). Grease and flour a 20x20cm square or rectangular baking pan. Alternatively, use parchment paper for easier unmolding.

Melt butter (using a double boiler or microwave) and let it cool slightly. Warm the milk.

Separate egg whites from yolks. In a bowl, vigorously whisk the egg yolks with granulated sugar, vanilla sugar (or lemon zest), and water until the mixture lightens and becomes foamy.

Incorporate the warm melted butter into the previous mixture. Then add the sifted flour and mix gently until a homogeneous dough is obtained.

Gradually pour in the warm milk, whisking gently to obtain a smooth, fluid batter.

Beat the egg whites with a pinch of salt into stiff peaks. Gently incorporate the egg whites into the batter using a spatula, lifting the batter from the bottom to the top to avoid breaking them. The texture will remain slightly lumpy, which is normal!

Pour the batter into the prepared pan. Bake for 50 to 60 minutes, until the top is golden brown and the cake is firm to the touch. Baking time may vary depending on your oven.

Let the cake cool completely in the pan before refrigerating for at least 2 hours. Before serving, dust with powdered sugar. Cut into squares and serve cold.