* Preheat the oven to 170°C/150°C Fan/325°F. Line a baking sheet with baking parchment.
* Melt the butter and let it cool a bit. Put the brown and white sugars into a bowl, pour the slightly cooled, melted butter over them and beat together.
* Beat in the vanilla, the cold egg and cold egg yolk until your mixture is light and creamy.
* Slowly mix in the flour and bicarb until just blended, then fold in the chocolate chips.
* Scoop the cookie dough into an American quarter-cup measure or a 60ml/quarter cup round icecream scoop and drop onto the prepared baking sheet, plopping the cookies down about 8cm/3 inches apart. You will need to make these in 2 batches, keeping the bowl of cookie dough in the fridge between batches.
* Bake for 15–17 minutes in the preheated oven, or until the edges are lightly toasted. Cool on the baking sheet for 5 minutes before transferring to wire racks.