Preheat your oven to 355°F/180°C in fan-assisted mode. Use a digital kitchen scale to weigh the eggs in their shells and, in separate bowls, add the same weight of butter and flour as the eggs. For the sugar, use the weight of the eggs as a basis, but subtract 10 g (example - if the eggs are 230 g, use 220 g of sugar).

Melt the butter in a micro-wave safe bowl and set aside.

Mix the flour, sugar, baking powder and vanilla sugar in a bowl.

Make a well with your hands. Add the eggs and beat lightly. Add the melted butter and mix gently one last time, until you obtain a homogeneous texture.

Butter your mould, add a little flour to the bottom and edges, then pour in the batter.

Place thin slices of butter in the middle of the cake and bake for 40 to 50 minutes, watching regularly. To be sure the cake is perfectly baked, stick a knife into the center. If it comes out almost clean, with no liquid batter (but a few crumbs), it's done! Leave to cool completely before unmolding.