Preheat oven to 350°F. Line a 12x16 inch baking sheet with parchment paper.
Cream together butter, sugar, and vanilla sugar until light and fluffy. Add eggs one at a time, mixing well after each addition.
In a separate bowl, whisk together flour, cocoa powder, and baking powder. Gradually add the dry ingredients to the wet ingredients, alternating with the milk, and mix until just combined.
Spread the batter evenly in the prepared baking sheet and bake for about 25 minutes, or until a toothpick inserted into the center comes out clean. Let it cool completely.
Meanwhile, for the pudding layer, bring 3 cups of milk to a boil in a saucepan.
In a separate bowl, whisk together the pudding mix, sugar, and the remaining half a cup of milk until smooth.
Gradually whisk the pudding mixture into the boiling milk, stirring constantly until it thickens. Remove from heat and stir in the chopped dark chocolate until melted and smooth. Then, let it slightly cool.
In another bowl, beat the heavy cream until stiff peaks form. Now, gently fold the whipped cream into the cooled pudding.
Spread the pudding mixture evenly over the cooled cake and smooth the top.
Refrigerate for at least 2 hours, or until the pudding layer is set.
Before serving, crush the chocolate cookies and sprinkle them evenly over the pudding. Finish by topping with the grated dark chocolate.
