For the nut sponge cake, grease and flour the mold. Beat the yolks, add the sugar and vanilla sugar. Spoon in the hot water until thick and fluffy.
Very important: the mixture must almost double in size and become much lighter or almost white.
Beat the egg whites with a little salt and a little cold water until stiff.
Stir rum and hazelnuts into the yolk mixture, lastly carefully fold in flour and snow (a little flour and a little snow, then repeat). Pour into the pan and bake in the preheated oven for 35-40 minutes at medium heat.
Turn out the nut sponge cake and sprinkle with powdered sugar.