Grind petit beurre (or similar biscuit) into coarse flour. Grind walnuts as well.

Place sugar in a deep pot and cover with 16 tablespoons of water. Cook on low and stir frequently until it boils. Continue boiling for exactly 2 minutes, and remove off heat.

Separately, combine dark chocolate (broken up or in chips) with milk and butter in another pot. Melt on low and stir frequently. When it melts remove off heat.

Pour ground biscuit into the boiling water and mix thoroughly with a spatula. Slice the butter in, and continue mixing. Finally, add the walnuts, and stir once again until the mass is completely combined.

Transfer the marzipan mass into a 12x18 inch pan, and use your palms to spread it evenly in the pan (be careful not to burn your fingers).

Pour the chocolate topping over it and also spread evenly over the entire marzipan. Leave aside to cool down. Cut into approximately two inch squares.