Break the chocolate into pieces and melt it together with the butter in a double boiler, constantly stirring with a spatula or a wooden spoon. Remove the resulting thick chocolate sauce from the double boiler and let it cool.

Meanwhile, combine the eggs with 100 g of brown sugar: crack the eggs into a separate bowl and whisk them, gradually adding the sugar. You can whisk with a mixer or by hand—whichever you prefer—but do so for at least two‑and‑a‑half to three minutes.

Chop the walnuts on a cutting board with a sharp knife. You can pre‑toast them in a dry skillet until they become aromatic, but that's optional.

Carefully add the remaining sugar to the cooled melted chocolate with butter, then add the flour and ground nuts, and mix everything well with a whisk.

Then pour in the sugar‑egg mixture and thoroughly combine it with the chocolate mass. The batter’s color should be uniform, without streaks.

Preheat the oven to 200 °C. Line the bottom of a small, deep, heat‑proof pan with parchment paper or a silicone mat. Pour the batter into the pan. Place it in the oven and bake for twenty‑five to thirty minutes until a sugary crust forms.

Take the finished cake out of the oven, let it cool, and cut it into squares with a sharp knife or a pizza cutter — the pieces will turn out especially even.

You can serve brownies plain, dust them with powdered sugar, or arrange the squares on plates and garnish each serving with a scoop of vanilla ice cream.