Separate the eggs; whip the 4 egg whites into a stiff foam.
Fold in the 4 egg yolks one by one, then the sugar and the ground hazelnuts, with gentle movements so the foam does not deflate.
Pour the mixture into a buttered, floured ring mold. Cover the mold, set it in a larger pot with already-boiling water reaching halfway up the mold, and steam over low heat for about 35-40 minutes, until the top springs back when touched.
Meanwhile, melt the chocolate with the milk over low heat, add the vanilla sugar, and cook, stirring constantly, into a thick cream (about 8-10 minutes); remove from the heat and let cool completely.
Turn the cooked soufflé out onto a plate carefully, shape a small hollow in the center, and pour in the cooled chocolate cream.
Whip the 5 egg whites into a stiff foam, mix in the vanilla powdered sugar, and coat the whole soufflé evenly with this foam.
Place in an oven preheated to 150 °C, and dry out for about 12-15 minutes (do not let it brown), until the coating is slightly dry on the outside but still soft inside.
Serve immediately, while the foam is light and the inner cream still warm.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.