In a bowl, mix together your flour, baking soda, cornflour and salt. Set aside.
In a large bowl, whisk together the butter and the two sugars until thoroughly combined. Add the egg, egg yolk and vanilla extract and whisk again until combined. Add the flour mixture and mix together using a wooden spoon. Finally pour in your chocolate chips, reserving 1 tbsp of them to put on top later. Mix everything together so you get an even distribution of chocolate chips. Then place some clingfilm over the bowl and chill in the refrigerator for at least 2 hours (and up to 3 days). It’s very important to chill the dough, so you don’t get flat cookies that spread all over your baking tray upon baking.
When you’re ready to bake the cookies, pre-heat the oven to 160c/325f. Line 3 large baking trays with baking parchment or a silicone baking mat. Take the cookie dough out of the refrigerator and scoop out a heaped tablespoon full (you may want to weigh out the dough to get even sized cookies. 2 oz/55g of dough makes a good sized cookie. You’ll get about 16-18 cookies out of this). Roll the dough into a ball with your hands, and place on you baking sheet. Repeat until you have used all of the dough. Make sure your dough balls are a good distance apart on the baking sheet as they will spread. I tend to put 6 or 7 cookies on each baking sheet.
Place in the oven and cook for 10-13 minutes, depending on whether you like your cookies soft and chewy or a bit more crisp. Take out of the oven and push the reserved chocolate chips gently into the hot cookies. Leave to cool on the baking tray for 10 minutes before gently lifting off with a spatula and cooling completely on a cooling rack.