Preheat oven to 180C / 350F /160C fan forced. Line 2 large baking trays with baking paper.
Sift together the flour, cornflour, cocoa and baking soda
In a separate bowl, beat together the butter and both sugars until lightened and fluffy. Scrape down the sides of the bowl and add half the flour mix. Beat until just combined. Add the milk and remainder of the flour and mix until it just starts to come together.
Finally, add the choc chips and salt and give the mixture another beat until mixed through and the dough is clumping
Pull the dough together with your hands and then use a medium ice cream scoop to scoop the mixture and transfer to the baking trays. Space the cookies about 1-2 inches apart.
Sprinkle a little more sea salt over the top of each one.
Bake for around 16 minutes, turning the trays halfway through.
Allow them to cool on the trays for a few minutes before using a spatula to help to transfer them to a cooling rack. Once cool, they will firm up and be crispy on the outside and chewy in the middle
I use a standard Australian 20ml tablespoon (4 teaspoons worldwide)
For best results, you should always weigh ingredients like flour and sugar. Kitchen scales like these are relatively cheap but if you can’t weigh the ingredients, use the spoon and level method (don’t scoop).