Grease a springform pan (26 cm Ø) and dust with flour. Coarsely chop the walnuts (not fine!). Mix with 150 g flour, 75 g sugar and 1 pinch of salt in a bowl. Melt 125 g butter at low heat.
Pour immediately into the flour mixture, kneading everything with the dough hooks of the hand mixer. Knead in 1 egg yolk and then briefly knead into crumbles with your hands. Sprinkle 1 tablespoon of flour on top and shake vigorously.
Break the chocolate into pieces and melt them with 150 g butter in pieces in a hot water bath. Leave to cool for about 10 minutes.
Mix 200 g flour, 200 g sugar, vanilla sugar and 1 pinch of salt in a bowl. Add the eggs and chocolate butter mixture and mix everything briefly with a hand mixer until the dough is smooth.
Spread the dough into the mould. Sprinkle nut crumbles on top and grate coarsely between your fingers. Bake in the preheated oven (electric range: 175 °C/circulating air: 150 °C/gas: level 2) for approx. 40 minutes. Let it cool down and dust with icing sugar