Prepare the Oven and Pan: Preheat the oven to 180°C (350°F), or 160°C for a fan-assisted oven. Grease an 8 × 8-inch square baking pan and line it with baking paper, leaving enough paper to help lift the brownies out after baking.
Melt the Butter: Place the butter in a large saucepan and warm it over low to medium heat until completely melted.
Incorporate the Sugars: Add the granulated sugar and packed brown sugar to the melted butter. Stir continuously over the heat for about 5 minutes, allowing the sugars to partially dissolve.
Melt the Chocolate: Remove the saucepan from the heat. Add the finely chopped dark chocolate and stir until completely melted and no chocolate lumps remain. The mixture may look slightly grainy at this stage, which is normal.
Cool the Chocolate Mixture: Set the saucepan aside for 10 minutes to allow the butter, sugar, and chocolate mixture to cool slightly before adding the eggs.
Add the Eggs and Vanilla: Add the room-temperature eggs and vanilla extract to the cooled chocolate mixture. Whisk until everything is fully incorporated and the mixture looks smooth.
Combine the Dry Ingredients: Sift the plain flour, unsweetened cocoa powder, and table salt into the chocolate mixture. Switch to a silicone spatula and gently stir until the dry ingredients are evenly incorporated, taking care not to overmix.
Fold in the Hazelnuts: Add the chopped toasted hazelnuts to the batter, reserving a small handful for the topping. Fold gently until the nuts are distributed throughout.
Fill the Baking Pan: Transfer the brownie batter into the prepared pan and spread it evenly. Spoon small dollops of Nutella over the surface, then use a knife to swirl the spread through the top of the batter.
Add the Hazelnut Topping: Scatter the reserved chopped hazelnuts over the Nutella-swirled surface, distributing them as evenly as possible.
Bake the Brownies: Place the pan in the preheated oven and bake for approximately 40 minutes. Check the center with a toothpick or wooden skewer; it should emerge with moist crumbs attached. Avoid overbaking to preserve the brownies' fudgy texture. Cool in the Pan: Remove the brownies from the oven and leave them in the pan for 10 minutes. This brief resting period allows them to settle before being transferred.
Transfer to a Cooling Rack: Use the overhanging baking paper to lift the brownies carefully out of the pan. Place them on a wire cooling rack and allow them to cool to room temperature or slightly above before slicing.
Slice and Serve: Cut the cooled brownies into 16 portions and serve. For the neatest slices, allow the brownies to cool sufficiently so their soft centers hold together when cut.
