Melt the butter and fold in the flour.
Blend in the hot milk with the vanilla, eggs and egg yolks.
Pass through a sieve and beat the batter until airy.
Fold the egg whites whipped with the sugar, little by little.
Spread the batter across the 60 x 40 cm baking sheets (875 g batter per sheet).
Bake at 150°C for 15 minutes.