Preheat the oven to 350 degrees.
Mix butter, sugars, vanilla, and egg yolk until combined.
Stir in flour, cornstarch, baking soda, and salt. When a soft dough has formed, stir in the chocolate chips.
Roll the dough into two balls and place on a baking sheet – there should be plenty for two large cookies plus a little bit of dough for a snack.
Bake for 10-12 minutes, depending on your oven temp and desired level of gooey cookie center.
Rest the cookies for a few minutes so they settle down and solidify a bit.
