Melt the butter and let it cool down a little. Roughly chop the chocolate and walnuts. Mix flour, sugar, 50 g cocoa, baking powder and vanillin sugar in a bowl. Beat the eggs in a mixing bowl with the whisk of the hand mixer until creamy.
Allow butter to run in and stir in. Stir in the flour mixture well with a mixing spoon. Stir in crème fraiche and fold in chocolate and nuts in portions. Put the dough into a greased springform pan (26 cm Ø) and smooth it down.
Peel, quarter and core apples and carve the roundings several times with a knife. With the rounding facing upwards, place the apples close together in the dough. Bake in the preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) on the middle shelf for approx. 45 minutes.
In the meantime warm up the jam. After 25 minutes of baking time, remove the cake from the oven. Brush the apples with jam, put them back in the oven and finish baking. Remove from the oven, place on a cake rack, remove from the edge of the springform pan and allow to cool in the pan.
Mix 1/2 tsp. cocoa and icing sugar. Remove the cake from the springform pan, place on a cake plate and dust the edge with icing sugar and cocoa mixture.