Preheat oven to 180°C and line 2-3 baking trays with baking paper.
Using the stand mixer fitted with a paddle attachment, beat the butter for approximately 1 minute on medium speed until smooth and creamy. Then add both the brown and white sugar and beat on medium-high speed until the mixture is light in colour and fluffy.
Beat in egg, egg yolk, and vanilla on high speed until combined before adding in ¼ cup of Nutella that has been stirred and beating until just combined.
Whisk the flour, cornflour, baking soda and salt together in a large bowl. On low speed, slowly mix into the wet ingredients until combined.
Remove the bowl from the stand mixer and add in the chocolate chips until evenly disbursed. Stir the remaining nutella then add to the mixture until just combined. There is no need to mix in well in this step, but note the mixture will be a little crumbly.
Roll approx. 1 tablespoon of mixture into balls then place on the baking tray. No need to flatten the balls into a biscuit shape.
Bake the biscuits for 10 minutes, until golden brown around the edges and the middle of the biscuits still a little soft. Leave to cool for 5 minutes on the tray before transferring to a cooling rack.