Preheat the oven to 200C/180C Fan/Gas 6. Line an 8" Square Pan.
Start by melting the dark chocolate: break up the dark chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn’t burn.
Combine the melted butter, granulated sugar and light brown sugar in a large bowl/bowl of a stand mixer – on a medium speed for 3-5 minutes.
Add in the eggs and vanilla and mix again. Then, checking first that the chocolate isn't so hot as to cook the eggs, add it in and keep mixing.
Add the flour and cocoa powder, as well as the nuts (reserve some to scatter at the end) and chocolate, and fold in gently.
Pour the batter into the prepared tin. Loosen the Nutella by placing in a microwavable bowl and heating for about 20 seconds. Drizzle this over the brownie batter and swirl in using a spoon. Scatter over any remaining hazelnuts.
Bake for 30 minutes. Let cool before slicing and serving.