1. Preheat oven to 350°F (175°C). Grease two 9-inch (23 cm) round cake pans and line bottoms with parchment.

2. In a large bowl, sift together cocoa powder, flour, and a pinch of salt.

3. Add granulated sugar, vanilla sugar, and softened butter. Whisk until smooth and glossy.

4. Beat in eggs one at a time, mixing thoroughly after each addition, until batter is rich and silky.

5. Divide evenly between the prepared pans and bake for 30 to 35 minutes, or until a skewer inserted in the center comes out clean.

6. Cool in pans for 10 minutes, then turn out onto wire racks to cool completely.

7. For the topping, mix 180 g (1 1/2 cups) flour with 115 g (1/2 cup) cold cubed butter. Rub together with fingertips until coarse, crumbly clumps form.

8. Press mixture into the bottom and slightly up the sides of a 9-inch (23 cm) springform or cake pan.

9. Bake for 20 to 25 minutes, until golden and firm to the touch. Let cool completely.

10. Heat cream in a saucepan until just simmering. Remove from heat, add chopped milk chocolate, and let sit for 2 to 3 minutes.

11. Whisk until smooth, glossy, and fully melted. Let cool slightly until spreadable.

12. Place one cooled cake layer on a serving plate. Spread a thick layer of ganache over the top.

13. Set the second cake layer on top. Sprinkle the cooled biscuit topping generously over the surface, pressing lightly to adhere.

14. Serve immediately for the best contrast of textures. Store leftovers covered at room temperature for up to 2 days.