In a heatproof bowl put dark chocolate pieces and soft butter; place over the saucepan and stir regularly until melted and very smooth. Just make sure bottom of the bowl doesn’t reach the water. Remove from the heat and let cool for a bit.
In a large bowl whisk eggs; add brown and white sugar, vanilla sugar and cinnamon. To the egg mixture stir in cooled chocolate and flour. Transfer batter to the baking tray; bake for 30-40 minutes.
When ready leave it to cool before cutting in to squares. Dust with confectioner sugar.