For the grillage casserole, melt the sugar, preferably in a copper pan, over low heat, brown until golden, add the hazelnuts and toast briefly.
Brush a baking tray with oil, empty hazelnuts on it while still very hot, let cool and grate finely (grillage).
Bring Mich, sugar, THEA, salt and vanilla sugar to a boil, quickly add sifted flour and stir until the mixture separates from the pot. Cool, gradually stir in yolks.
Beat egg whites with sugar until stiff and fold into yolk mixture together with grated grillage.
Grease and flour the baking dish, spread the grillage mixture in it and bake in the preheated oven at 160 degrees for about 50-60 minutes.
Sprinkle the grillage casserole with powdered sugar before serving.