Preheat oven to 350 degrees. Line cookie sheet with parchment paper.
In a large mixing bowl, mix butter and sugars on medium until light and fluffy.
Add in the egg and vanilla and mix on medium until completely combined.
Add in the flour, cornstarch, and baking soda. Mix on low until the flour is incorporated. If you have your mixer on medium or high, I promise you'll cover everything with flour.
Using a cookie scoop, scoop the cookies onto the baking sheet. I did 3 rows of 4 on my baking sheet.
Place in the oven and bake for 10 minutes or until lightly golden brown. My large cookies took 10 minutes and my itty bitty cookies took about 8. So, make sure you keep an eye on these.
Remove from oven and let cool for 10 minutes. Place on a cooling rack to finish cooling.