Coarsely chop the couverture and chocolate. Melt both with 125 g butter on a warm water bath. Coarsely chop the walnut kernels. Pour both masses into bowls. For the white couverture, add 200 g sugar, 1 packet of vanillin sugar, 1 pinch of salt, 3 eggs, 160 g flour and half the baking powder.
Mix to a smooth dough with the whisk of the hand mixer. Add the remaining ingredients to the dark chocolate and also work into a smooth dough. Now spread both masses in patches on an oven pan lined with baking paper (32 x 39 cm).
Using a fork, pull 1 x through the distributed mass in a snake-like manner. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 30 minutes. Remove and let cool on a cake rack.