Preheat the oven to 180°C (350°F) and line two baking trays with baking paper.
With an electric mixer, beat the cold butter with the brown sugar and sugar creaming them together.
There should still be clumps of butter visible.
Add the eggs and vanilla extract and beat in. Again, there should still be chunks of butter visible.
In a separate bowl, mix the flour, cornflour and baking powder together.
Add the flour mixture to the wet ingredients. The batter will be quite stiff.
Add the chocolate chips and stir in.
Dollop the Nutella around the batter and fold through a few times. You want it swirled not fully incorporated.
Divide the mixture into around 10-12 large balls and arrange on the prepared pan
Only five to a tray as they will spread.
I didn't roll mine, more like clumped them together in a rustic look. You want them tall balls rather than flatter balls as this will help them remain thick.
Chill the dough for 30 minutes – 1 hour to re chill the butter and avoid spreading.
Bake for around 15-20 minutes. The dough will still be a little soft to the touch.