Set a large heat-proof bowl over a saucepan of simmering water to create a bain-marie. Add butter and break the 200g dark chocolate into small pieces to bowl. Heat, stirring occasionally until the chocolate and butter are completely melted and smooth. Remove bowl from heat and let it cool down.

Beat the eggs with the salt until foamy. Add sugar, vanilla extract, the mix of chocolate and butter, the flour and walnuts (walnuts are optional)

Pour batter into prepared pan. Bake in preheated oven in 180° for 25 minutes if you want it to be moist and 30 if you want it to be dry. (I prefer 30 minutes)
Let it cool down and enjoy!