Preheat oven to 200°C/395°F fan-forced (or 220° without fan) and line 2 cookie sheets with baking paper
In a bowl, combine the flour, cornflour, salt, baking soda and baking powder. Use a whisk to ensure they are completely combined (or you can sift the ingredients). Set aside
In the bowl of a stand mixer and using the paddle attachment, beat butter and sugars on medium, until well blended
Add the eggs, one by one and continue to mix on low until combined *see notes
Turn the mixer off, then add the combined flour ingredients. Turn the mixer on low and mix until almost combined
Add both chocolate chips and mix on low until almost combined. Remove the bowl and use a rubber spatula to mix the ingredients, ensuring the chips are distributed throughout the dough
(I use food gloves for this) pull out mounds of dough and place each mound onto lined scales. You want each dough ball to weigh 170g (6oz). Without rolling the dough (see notes), place it onto a lined cookie sheet, 4 cookies per sheet.
Bake both trays in the preheated oven for 8 minutes *see notes
Once cookies are ready, remove the trays from the oven and slide the cookies and baking paper off the tray. Cookies will be soft in the middle, so leave them to cool before lifting them