Partially melt the butter in a microwave proof bowl, then finish melting it by mixing. It should look milky rather than clear yellow, and it will be cooler so the cookie dough can be rolled without refrigerating.
Mix in both the sugars, then the yolk, vanilla and salt. Scatter flour and baking soda across surface, mix until flour is almost mixed in, then stir in MOST of the choc chips (reserve some for topping).
Shape into 2 x 1.5cm/0.6” thick discs. Bake 13 – 14 min at 190°C/375°F (170°C fan) or until edges are golden and middle is pale gold. Top with reserved choc chips and sprinkle with salt flakes (if using), leave to cool on tray for 10 min then attack!
ONE GIANT COOKIE - Shape into one 2 cm / 0.8” thick disc, bake at 180C/350F (160C) for 16 - 18 minutes until golden on edges.
