Melt 250 g of dark chocolate with 150 g of butter in a bain-marie or with the microwave.
Work the eggs with the sugar. Coarsely chop 160 gr of pistachios and add them to the eggs.
Add the molten chocolate and vanillin. Mix very well, put in the square cake tin and bake for 60 minutes at 160 degrees. After baking, let it cool.