Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper.
In a double boiler or microwave-safe bowl, melt the dark chocolate and butter together. Stir until smooth and set aside to cool slightly.
In a mixing bowl, whisk together the sugar, eggs, and vanilla extract until pale and frothy.
Gradually mix the melted chocolate into the egg mixture, stirring until well combined.
Sift the flour, cocoa powder, and salt into the wet ingredients. Fold gently until just combined.
Fold in the hazelnuts, reserving a few for sprinkling on top if desired.
Pour the batter into the prepared pan, spreading it evenly. Sprinkle reserved hazelnuts on top.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Let the brownies cool completely in the pan before slicing into squares and serving.