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.