Prepare the Base: Start by crushing the cookies into fine crumbs. You can use a food processor or place them in a plastic bag and crush them with a rolling pin. Set the crumbs aside.
Make the Base Mixture: In a medium saucepan, melt the butter over low heat. Stir in the milk, sugar, and vanilla sugar. Gradually add the cocoa powder while stirring until fully combined and smooth.
Combine the Base Ingredients: Remove the saucepan from heat and stir in the crushed cookies and chopped walnuts. Mix until everything is well combined into a thick, sticky mixture.
Form the Base Layer: Press the cookie-walnut mixture into the bottom of a greased or lined cake pan. Use a spoon to press it down evenly to create a firm, solid base. Chill the mixture in the refrigerator for about 10–15 minutes to allow it to set.
Prepare the Top Layer: After the base has set, sprinkle the powdered sugar over the top and garnish with the remaining chopped walnuts.
Chill and Serve: Allow the cake to chill for at least 30 minutes in the fridge to firm up and allow the flavors to meld together. Slice and serve!