Preheat the oven at 180 C degrees.
Line a pan with butter paper and lightly grease.
In a large microwave safe bowl melt together the butter and dark chocolate in 30 to 45 second, stirring inbetween, until the mixture is mixed together well.
Whisk in the sugar, followed by the vanilla extract. Stir in the Nutella (for easier stirring, microwave for 30 seconds before).
whisk the eggs
Carefully stir in the flour an salt. Pour the batter into the prepared pan and bake for 30-35 minutes or until the top looks set and an inserted toothpick comes out clean or with a few damp crumbs.
Cool fully to room temperature, or for at least 30 minutes.
Cut into square and enjoy