In a bowl, separate your yolks and your whites and keep your whites to whisk into a meringue.
Melt your chocolate and butter together and leave to cool fully before adding your egg yolks, salt and cocoa powder.
In a separate bowl, mix together your egg whites. As these are duck eggs, they will take longer to whisk but keep at it. It's best to use an electric whisk here and about 2 minutes in, add in your granulated sugar little by little and whisk until stiff peaks appear.
Move over to your chocolate mix and fold in 1/3 of the egg white and fold slowly but do not whisk as you'll whisk the air bubbles out of the meringue.
Fold in the remaining egg white and pour into a brownie tin and cook for 25 minutes.