1. Preheat the oven to 400°F or 200°C.

2. In a separate bowl, combine the flour, baking powder, and vanilla sugar, then add the cubed butter and mix until the mixture resembles coarse crumbs.

3. Press the mixture into a long pan, about 12x10 inches or 30x25 cm in size.

4. Use a fork to create a decorative edge around the mixture in the pan.

5. Bake in the preheated oven for approximately 10 minutes, or until the edges are lightly golden.

6. While the crust is baking, prepare the walnut mixture by combining the granulated sugar, chopped walnuts, and a pinch of salt in a saucepan.

7. Cook the walnut mixture over low heat, stirring occasionally, until the sugar has dissolved and the mixture is well combined.

8. Remove the saucepan from the heat and set the walnut mixture aside.

9. Once the crust is ready, spread the walnut mixture evenly over the top of the crust.

10. Return the pan to the oven and bake for an additional 15 minutes, or until the top is lightly golden.

11. Allow the cake to cool completely before cutting it into squares and serving.