Prepare the Baking Pan: Preheat the oven to 350°F (175°C). Line a 9 × 13-inch baking pan with parchment paper and set aside.
Combine the Dry Ingredients: In a large mixing bowl, whisk or sift together the sugar, flour, unsweetened cocoa powder, and salt until evenly blended.
Mix the Butter and Hazelnut Paste: Stir the melted butter and hazelnut paste together until combined. Pour the mixture over the dry ingredients and stir to incorporate.
Add the Eggs and Vanilla: Add the eggs and vanilla to the bowl. Mix until the ingredients are combined into a uniform brownie batter.
Spread the Batter: Transfer the batter to the prepared baking pan. Use a spatula to spread it evenly across the pan.
Bake the Brownies: Bake at 350°F (175°C) for 30 minutes. Remove the pan from the oven and allow the brownies to cool completely before adding the ganache.
Melt the Butter: Place the butter in a small skillet over heat and let it melt.
Toast the Hazelnuts: Add the coarsely chopped hazelnuts to the melted butter. Sprinkle with a pinch of flaky sea salt and stir until the nuts begin to brown.
Cool the Hazelnuts: Remove the skillet from the heat and set the hazelnuts aside to cool.
Prepare the Double Boiler: Place a medium heatproof bowl over a small saucepan of simmering water, ensuring the bowl sits above the water.
Warm the Chocolate and Cream: Add the chocolate and cream to the bowl. Let them warm over the steam for a few minutes, until the chocolate begins to melt.
Stir the Ganache: Stir the chocolate and cream until the mixture becomes smooth.
Add the Hazelnut Paste: Remove the bowl from the heat. Stir in the hazelnut paste until the ganache is smooth and fully combined.
Top the Brownies: Pour the ganache over the completely cooled brownies and spread it evenly across the surface.
Add the Buttered Hazelnuts: Sprinkle the cooled buttered hazelnuts over the ganache, followed by a little flaky sea salt, such as Maldon.
Let the Ganache Set: Allow the ganache to firm up before cutting the brownies into squares.
Serve the Brownies: Slice the set brownies into portions and serve, enjoying the rich chocolate base, smooth hazelnut ganache, and crunchy toasted hazelnut topping.
