Preheat your oven to 356°F / 180°C and line a square brownies pan with parchment paper.
In a bowl, whisk together the eggs and sugar for 1-2 minutes, then add the flour and salt and set aside.
Melt half of the butter with the semi-sweert chocolate cut into pieces in a double boiler and set aside in a bowl.
Do the same with the white chocolate and the other half of the butter and place this preparation in a container.
Divide the first egg-sugar-flour mixture equally between the two containers containing the two chocolates and add the crushed nuts to the two preparations, one half with the dark chocolate brownies batter and the other half with the white chocolate brownie batter.
Pour the dark chocolate mixture into the brownies pan first and pour the white chocolate mixture on top.
Using the tip of a knife, make swirls in the batter to give this marbled pattern.
Bake for about 25-30 minutes.
Let the brownie cool completely in the pan before removing from the pan and cutting.