Preheat oven to 175 degrees celsius (fan-forced). Grease and line a 20cm square cake/slice tin with baking paper.
Place the chocolate and chopped butter into a microwavable 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 and stir through until completely combined.
Add the brown sugar and vanilla essence and beat well.
Add in the 2 whole eggs and 1 egg yolk and beat well.
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 (this step is not essential but will make it far easier to slice into neat pieces).