For the dough, mix butter with brown sugar, 1 packet of vanilla sugar and salt until creamy. Stir in the egg. Mix flour, hazelnuts, baking powder and oat flakes, stir into the fat cream. Grease a springform pan (26 cm Ø), put the soft dough into it and press a flat bottom with floured fingers, forming a rim about 4 cm high.
Chill for about 1 hour.
Wash the gooseberries, drain well and clean. Heat the jam in a wide pot. Add gooseberries, simmer for 4-5 minutes. The gooseberries should remain whole and not burst open.
Remove from the heat and let it cool down. Spread gooseberries on the floor. Bake in the preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 40-45 minutes on the bottom shelf.
Let it cool down on a cake rack.
Coarsely chop the hazelnuts. Roast in a pan without fat. Sprinkle with sugar and caramelize while turning. Spread on a piece of baking paper and let it cool down. Whip the cream until stiff, sprinkle with 1 sachet of vanilla sugar.
Spread some cream in the middle of the cake. Sprinkle the cake with nuts. Dust the edge with icing sugar. Add the rest of the cream.