Crush 400g of biscuits into small pieces and set them aside.
Grind 100g of walnuts finely, leaving some for garnish.
In a saucepan, heat 200ml of milk over medium heat.
Add 50g of sugar, 10g of vanilla sugar, and 50g of cocoa to the milk. Mix well and bring to a simmer.
Add 120g of butter to the milk mixture and stir until melted.
Remove the saucepan from heat and add 120g of chocolate. Stir until the chocolate is fully melted.
Pour the chocolate mixture over the crushed biscuits and add the finely ground walnuts. Mix thoroughly.
In a separate saucepan, heat 100g of cream until it starts to steam, then pour it over 40g of white chocolate and 1 tbsp of cream. Stir until smooth.
In a cake mold, alternate layers of the chocolate-biscuit mixture and the white chocolate ganache.
Refrigerate the cake for at least 2 hours until it sets.
Garnish with reserved walnuts.
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