Preheat the oven to 170°C. Line a 21 x 28cm metal brownie tin with baking paper on all sides.
Melt the butter and chocolate in a double boiler, then set aside to cool.
Beat the castor sugar, eggs, and vanilla extract together until light and fluffy and roughly doubled in volume.
Stir in the cooled chocolate and butter mixture.
Sift the flour and baking powder in a separate bowl, then add to the chocolate batter and briefly mix to combine.
Tip half the mixture into the prepared brownie tin. Dollop the Nutella in blotches evenly across the surface. Use a skewer to swirl each mound slightly into the batter.
Pour the remaining chocolate batter over the top and spread to distribute evenly.
Bake for 45 minutes. The brownies will still be a bit wobbly in the centre.
Leave to cool completely before removing from the tin and cutting.
If desired, spread a thin layer of Nutella on top and sprinkle with toasted hazelnuts.