Cream fat, sugar, vanillin sugar and salt with the whisk of the hand mixer. Separate the eggs. Stir the egg yolks into the fat mass one after the other. Mix flour and baking powder, sieve over it and stir in. Beat the egg white until stiff and fold into the dough. Remove 1/3 of the dough and mix with cocoa and milk.
Fold the walnuts into the light-coloured dough. Grease a box form (25 cm long; 1.5 litres capacity). Fill in half of the light-coloured dough. Put half of the dark dough as a blob on top and pull it lightly through the light dough with a fork. Spread the rest of the light-coloured dough on top. Place the rest of the dark dough on top as a blob. Bake in a preheated oven (electric: 175 °C/ gas: level 2) for about 1 hour. Cool down on a cake rack. Put the icing in boiling water and let it melt for about 10 minutes. Shake vigorously and pour over the cake. Brush smooth and let dry. Decorate with walnut halves as desired.
Bake in a preheated oven (electric: 175 °C/ gas: level 2) for about 1 hour. Cool down on a cake rack. Put the icing in boiling water and let it melt for about 10 minutes. Shake vigorously and pour over the cake. Brush smooth and let dry. Decorate with walnut halves as desired. Cut the cake open and serve, possibly decorated with mint. Results in about 14 pieces