Preheat the oven to 350F, then line a baking sheet with parchment paper or silpat.
In a medium bowl, cream the butter and both sugars together. I did so using my spatula.
4 tbsp butter, 1/4 cup brown sugar, 2 tbsp granulated sugar
Add in the vanilla and egg yolk and mix everything is smooth and well incorporated.
1 yolk, 1 tsp vanilla extract
Add in the dry ingredients over the wet ingredient mixture, starting with the flour, corn starch, baking soda, and salt. Stir just until combined.
2/3 cup flour, 1 tbsp corn starch, 1/4 tsp baking soda, 1/4 tsp salt
Fold in the chocolate chips, then using a medium cookie scoop, scoop out the cookie dough into 8 2-tablespoon cookie dough balls and place on the prepared baking sheet. ( READ NOTES FOR LARGER COOKIES)
1/2 cup semi sweet chocolate chips
Place them on the prepared baking sheet and bake for 8 minutes. Don't worry if they look underdone or gooey in the middle. That's what we want, they will continue to bake even after you pull them out of the oven to cool.
Top the cookies with extra chocolate chips as soon as they come out of the oven, cool on the cookie sheet for 5 minutes then transfer to a cooling rack.
Video
Notes
To measure out the flour, use the spoon and sweet method. Spoon the flour into the measuring cup and sweep the excess off with a flat knife or spatula. This will give you a more accurate measurement next to weightings. (USE A KITCHEN SCALE IF YOU HAVE)
The tops will be puffy when you pull these cookies out of the oven but it's okay because they will deflate as they rest and cook a little longer on the baking sheet while cooling.
Making the four large cookies? Use ½ tablespoon of cornstarch instead of a full tablespoon. A bigger cookie holds more moisture on its own, and the full amount makes the centers too soft to set properly at this size.They'll come out puffy, and that's normal, they settle as they cool. If you want them flatter, press the tops gently with your hand right after they come out, or press a few extra chocolate chips into the top. That downward pressure spreads them a little more and makes them look better too.
Bake 4 larger cookies
scoop out the cookie dough using a 1/4 cup large cookie scooper.