No need to preheat the oven yet, the dough needs to chill. Place flour, cocoa powder, cornflour, bicarbonate of soda, salt and chocolate chips into a medium sized bowl and stir to combine.
Place butter and sugars into a large bowl, or the bowl of your stand mixer and beat until light and fluffy, around 2 minutes on medium speed.
Add in the eggs and vanilla extract, scraping down the sides as necessary, and mix on medium speed for 1 minute, or until well incorporated.
Tip in your flour mixture and mix on low until a soft dough forms.
Tip into the bowl you measured your flour in and cover with cling film. Place the dough in the fridge to chill for at least 6 hours, I just leave it overnight.
Once chilled, preheat the oven to 180C/350F and line 2 baking trays with a Silpat/silicone mat/grease proof paper.
Using a 1.5tbsp cookie scoop, place balls of dough 2 inches apart on the trays and press each ball ever so slightly with the palm of your hand, so they don't roll away!
Place in the oven for 9-11 minutes until the edges are firm to the touch, but the middles look soft. In the light of the oven, the whole cookie will look mostly matte, but softer in the middle.
Leave to cool on the trays completely before transferring to an airtight container to store. Salted Double Chocolate Cookies will keep at room temperature for 5 days.