For the sponge mixture, whisk butter or margarine with a hand mixer with whisk at highest speed until smooth. Gradually stir in sugar and vanilla sugar until a bound mass is formed.
Stir in the eggs bit by bit (each egg a little ½ minutes).
Mix the nuts with baking powder and stir in portions at medium speed. Fill the dough into a springform pan (26 cm, bottom greased and covered with baking paper) and smooth it down. Slide the mould on the grid into the oven.
Top/bottom heat: about 180°C (preheated) Hot air: about 160°C (not preheated) Gas: level 2-3 (not preheated) Baking time: about 30 minutes
Remove the cake base from the mould, turn it over onto a cake rack and let the base cool down.
Cut eight well-chilled Roche in half with a sharp knife and put them aside for garnishing. Crush the remaining balls in a bowl.
Whip cream with cream stiffener until stiff and fold into the crumb mixture in 2 portions. Place a cake ring around the cake base and spread the mixture on the base. Put the cake in a cool place.
To decorate, coarsely chop the couverture and mix in a small pot in a bain-marie over low heat until smooth. Roast the hazelnut flakes in a pan without fat. Remove the cake from the cake ring and press the hazelnut flakes onto the cake edge.
Put the couverture in a freezer bag, cut off a corner, decorate the cake with the couverture and refrigerate.
Whip the cream with vanillin sugar until stiff, place in a piping bag with a star-shaped cover. Decorate the cake with cream tuffs and garnish with the halved Roche. Serve the cake well chilled.