Preheat the oven to 175 C. Line an 8" square pan non-stick baking paper, leaving a little overhang. If using a 'not non-stick baking paper', you will need to lightly grease both the paper and pan.
In a bowl placed over a pot of simmering water, melt the butter, chocolate and cocoa until you have a smooth mixture. If there are small lumps of chocolate, turn off the heat and stir to melt them completely, rather than overheating the chocolate. Let this cool while you get the rest of the batter ready.
Beat the eggs, sugar and vanilla in a large bowl till pale and thick, about 4 minutes. This is where you're ensuring a crust forms, so beat away! An electric mixer will get the best results.
Add in the Nutella and beat lightly to combine.
Now pour in the cooled chocolate mixture and beat again till combined.
Sift in the flour, baking powder and salt, if using. Fold slowly to combine. The batter is thick and will need some muscle work but be gentle.
Pour into the prepared dish, smoothen the top and drop the extra 1/4 cup of Nutella in dollops all over the surface. Take a toothpick and swirl it around, taking care not let it blend into the batter. You want the swirly design!
Bake for 20 minutes till a toothpick poked in the center comes out with only a few moist crumbs.
Allow the brownies to cool completely at room temperature before slicing. The crust will be a little less crackly than a standard brownie recipe that has more sugar, and the Nutella swirls make it harder to see, but I promise it's still a pretty crackly crust!
Best stored at room temperature in an airtight tin for 2 to 3 days in humid environments, after which they can be kept in the fridge for a week, or even frozen for a month. Happy baking!