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 the 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 the pan on a wire rack and cool completely before slicing and serving.
