Melt the butter and then let it cool.
Separate the egg whites from the yolks.
Mix the yolks well with the sugar, the vanilla sugar until it doubles in volume. Add the butter, flour and mix well. Then add the warm milk little by little until it is well incorporated.
Whisk the egg whites with a pinch of salt and incorporate them into the dough with a spatula, stirring slowly from the bottom up.
The resulting composition should be similar to sour cream.
Put in the tray lined with baking paper and put in the oven heated to 180 degrees for about 40 minutes.
When it is ready, take out the cake and leave it to cool. Sprinkle with powdered sugar.