Preheat oven to 325F (160C) and prepare cookie sheets by lining with parchment paper.
Combine butter and sugars in a large bowl. Stir well.
Add egg and vanilla extract. Stir to combine and set aside.
In a separate, medium-sized bowl, whisk together flour, cornstarch, baking soda, and salt.
Gradually add flour mixture to wet ingredients — stir well so that all the flour is absorbed.
Stir in chocolate chips.
Scoop dough by rounded 1 ½ teaspoon onto prepared cookie sheets, placing at least 1″ apart. I like to roll my cookies into balls after scooping them so that they bake very uniform and round, but this is not necessary.
Bake on 325F (160C) for 6-8 minutes — cookies may still seem slightly soft in the centers, that is OK, they will cook completely on the cookie sheets. Don’t over-bake or your cookies will be too hard.
If desired, gently press additional chocolate chips into the tops of the warm cookies.