Preheat oven to 175 degrees celsius (fan-forced).
Grease and line a 20cm square cake/slice tin with baking paper.
Place the dark chocolate and chopped butter into a microwave-safe bowl.
Heat on 50% power for 3 minutes (or until just melted) in 30 second bursts, stirring at each interval with a dry metal spoon.
When the butter and the chocolate have melted, add the Nutella, brown sugar, vanilla extract, 2 whole eggs and 1 extra yolk and mix together well with a spoon until completely combined.
Gently stir through the plain flour until just combined (being careful not to over-mix).
Pour the brownie batter into the prepared tin and bake for 25-30 minutes.
Remove the brownie from the oven and set aside to cool in the pan.
Place the cooled brownie into the fridge for 2-3 hours. Cut into pieces and serve.