Preheat the oven to 180°C / 350°F and grease or line a 21 cm×21 cm (8” x 8” ) pan with parchment paper and set aside.
Melt the butter and chocolate in a glass heat resistance bowl with the bain-marie method (place the bowl in a saucepan ⅓ filled with simmering water, without touching the bowl to the water). Once melted, take the bowl from the saucepan and whisk the mixture until combined well.
Add the sugar in the mixture and whisk the mixture until combined well. Then whisk in the eggs and vanilla for 5 minutes.
In a separate bowl, mix the cocoa powder, flour, salt and walnuts.
Gently fold the dry ingredients into the wet mixture. Do not over mix at this stage. The dry mixture does not have to mix completely (read above the tips and tricks for making the perfect brownies).
Spread the batter into the pan. The batter will be very thick.
Bake the brownies for 22-23 minutes. It is key to not overcook the brownies. To understand if the brownies are ready, insert a toothpick into the centre of the pan. If it's covered with wet batter, the brownies are not cooked. If there are only a few moist crumbs, the brownies are ready. Make sure you keep checking the brownies every 2 minutes until you have moist crumbs.
Remove the brownies from the oven and let them cool down for at least an hour before slicing them into squares.
After the brownies are completely cooled down, slice them into squares. Store them covered to avoid them getting dry. Brownies can be stored for up to 1 week if stored in a well-covered container at room temperature.