Preheat the oven to 350F degrees.
Line an 8x8 inch pan with aluminum foil and lightly grease, or line with parchment paper
In a large microwave safe bowl melt together the butter and dark chocolate in 30 to 45 second bursts, stirring after each, until the mixture is melted together.
Whisk in the sugar, followed by the vanilla extract.
Stir in the Nutella (for easier stirring, microwave for 30 seconds before).
Whisk in 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. Then cut into 16 squares. Optionally, frost with Nutella.