-
Preheat the oven to 350 degrees. Spray an 8-inch square baking dish with cooking spray and set aside.
-
Combine the chocolate and butter in medium a microwave-safe bowl and heat in 30-second intervals, stirring after each interval, until chocolate is melted and mixture is smooth.
-
Set aside to cool for 5 minutes.
-
Stir the eggs and sugar into the chocolate mixture, mixing well. Mix in the vanilla, baking soda, and salt until well combined.
-
Fold in the flour, until just combined (just mix until you no longer see streaks of flour in the brownie batter).
-
Fold in the hazelnuts.
-
Pour the batter into the prepared pan and smooth the top. Dollop the Nutella over the brownie batter and use a butter knife to swirl the two together.
-
Bake for 25 to 30 minutes, until the top of the brownies looks set. Set pan on a wire rack and cool completely before slicing and serving.