Grease the pan and, if you wish, line it with parchment paper (it's not essential).

Melt the chocolate and butter in a double boiler and set aside.

Beat the eggs and sugar with a mixer for about 7 minutes, until the mixture triples in volume, turns pale and creamy, and forms a ribbon.

Carefully fold in the melted chocolate in three additions using a spatula, working from bottom to top to keep the mixture light.

Add the sifted flour in the same way, then fold in 60 g (2 oz ) of chopped walnuts, reserving the rest for decoration.

Pour the batter into the pan, smooth the surface, and sprinkle with the remaining walnuts.

Bake for 20 to 30 minutes, depending on the desired texture. A toothpick inserted into the center should come out with a little batter for a fudgy center.

Let cool before cutting into squares.

Enjoy it cold or warm, according to your preferences.