Melt the dark chocolate and unsalted butter using a double boiler.

Lightly beat the eggs, then add white sugar, brown sugar, salt, and vanilla extract. Mix until slightly frothy.

Add the melted chocolate mixture to the egg mixture and combine well.

Sift cake flour and cocoa powder into the mixture, then fold until no dry ingredients are visible.

Line a 20cm square pan with parchment paper and pour in the batter.

Bake in a preheated oven at 180°C for 25 minutes. Let it cool slightly before removing from the pan.