Preheat oven to 350 °F (177 °C). Line two baking sheets with parchment.
Cream butter, brown sugar, and white sugar 2 min until fluffy.
Beat in egg, yolk, and vanilla until silky.
Whisk flour, cornstarch, baking soda, and salt; add to wet mixture. Blend just until streaks disappear.
Fold in chocolate chips (and add‑ins).
Scoop 1½‑Tbsp mounds 2 inches apart.
Bake 9–11 min—edges set, centers pale.
Cool on sheet 10 min; transfer to rack.
Notes
* Pull at 9 min for “half‑baked” gooey centers.
* For thicker cookies, chill scooped dough 30 min before baking.
* Freeze unbaked balls up to 3 months; bake from frozen at 325 °F for 12 min.