Preheat oven to 400°. Grease two cookie sheets.
In a medium bowl, stir together the flour, cornstarch, baking powder, salt, and baking soda.
Using either a stand mixer or hand mixer, cream the softened butter and both sugars together until light and fluffy, 3-5 minutes. Mix in vanilla and egg until combined.
Slowly add the flour mixture to the sugar mixture. Mix until combined. Add in the chocolate chips. (The dough may be rather stiff at this point; some of the chocolate chips may have to be worked in by hand in the next step.)
Roll the dough into balls. Place on greased cookie sheets. Bake for 10 minutes, or until the edges are just set but the middles still look gooey. (It's important to pull the cookies before they actually look done!)
Let cookies cool on baking sheets for at least 15 minutes. (During this time, the residual heat will finish baking the cookies.)