Start by lining a cake mold with plastic wrap or parchment paper for easy removal after the cake sets.
In a large bowl, mix the crushed vanilla cookies with the chopped walnuts. Set this aside.
Combine the milk, granulated sugar, cocoa powder, and vanilla sugar (or vanilla extract) in a saucepan. Heat the mixture on medium heat, stirring constantly until it is well-combined and the sugar has dissolved.
Add the butter to the saucepan and continue stirring until the butter is completely melted and the mixture is smooth.
Pour the warm chocolate mixture over the crushed cookies and walnuts, and mix well until every piece is coated.
Transfer the mixture to the prepared cake mold, pressing down firmly to create an even layer.
Melt the dark chocolate with the vegetable oil either in a double boiler or in the microwave, stirring frequently to ensure it’s smooth and glossy.
Pour the melted chocolate over the cake base, spreading it evenly with a spatula to cover the entire surface.
Refrigerate the cake for at least 4 hours, or until it is set and firm.
Once set, remove the cake from the mold and peel away the plastic wrap or parchment paper.
Slice and serve chilled for the best taste experience. Enjoy your sumptuous Chocolate & Walnut Delight Cake!