Beat the butter with the sugar until fluffy. Stir in the eggs one by one, fold in the hazelnuts and baking powder.
Insert the bottom of a springform pan upside down and grease it, pour in the batter and bake in a preheated oven at 175°C for 30 minutes, let it cool down afterwards. Crush the Rocher balls. Whip 400 g cream until stiff, add cream stiffener and vanilla sugar and fold into the Rocher mixture in 2 portions, spread on the cake base and chill the cake for approx.
30 minutes. Coarsely chop the chocolate coating and melt in a bain-marie. Carefully remove the cake from the mold. Press the hazelnut flakes to the edge of the cake. Put the couverture in a freezer bag and cut a small hole in the bag, then decorate the cake with the liquid couverture in the bag.
Chill again for about 10 minutes. Whip 200 g cream until stiff and decorate the edge of the cake with 16 tuffs. Then cut the 8 Rocher balls in half and place them on the cream puffs.