Preheat oven to 180c.
Grease and line a 20 cm (8") square baking pan with baking/parchment paper and set aside.
Whisk together plain flour and cornflour and set aside.
200 g plain flour (all-purpose flour), 1 tablespoon cornflour (cornstarch)
In a large bowl add melted butter, light brown sugar, granulated sugar and vanilla then mix to combine.
160 g salted butter, 150 g light brown sugar, 100 g granulated white sugar, 1 teaspoon vanilla extract
Mix in eggs one at a time.
2 eggs
Add flours and mix until almost combined, then add chocolate chips and continue mixing until just combined.
¾ cup chocolate chips
Dollop half of the Nutella on top of the dough in the bowl, then roughly spread it to cover at least half of the surface.
155 g Nutella
Fold the dough 1 to 2 times to incorporate the Nutella, don't mix any more as you want swirls and ribbons, not just Nutella flavoured dough.
Tip dough into the prepared pan (don't spread it yet), then dollop about half of the remaining Nutella randomly over the dough.
Spread the dough into the corners of the pan (I use a small silicone spatula) swirling through the dollops of Nutella as you go.
Spoon any remaining Nutella over the top of the dough then lightly swirl again with a skewer or a spatula.
Bake for 20 - 25 minutes, or until the edges are golden but the center is set but still slightly gooey.
Remove from the tin and allow to cool on a wire rack until firm enough to slice (see my notes).
Or - place in the fridge to set for 1-2 hours then slice (my preference) and serve cold.