Finely chop the dark chocolate.
Melt the butter and dark chocolate together using a double boiler.
Add sugar, eggs, and vanilla extract, then mix well.
Add flour and salt, mixing until well combined.
Pour the batter into a pan lined with parchment paper.
Bake in a preheated oven at 180 degrees Celsius for 25 minutes.
Let cool completely, then cut into serving-sized pieces.