In a medium mixing bowl, add brown sugar, white sugar, and butter. Cream until light and fluffy.
Add egg, baking soda, cornstarch, salt, and vanilla. Mix well.
Add half of the flour. Fully incorporate. Then add the remaining flour. Mix to fully incorporate making sure not to over mix.
Divide into 6 large balls of dough. Each cookie will weigh about 5 ounces.
Place on a baking sheet lined with parchment paper. Space dough balls evenly on the pan. Slightly press on the dough ball to make a more of a cookie shape.
Bake for 12 minutes or until the sides are set.
Remove from oven and lightly sprinkle with salt flakes.
Cool for a few minutes before moving the cookies to a cooling rack to fully cool.