1cupdry rolled oatsalso known as "old-fashioned oats"
Preheat the oven to 350 degrees F. Line a sheet pan with parchment paper.
In a medium bowl, whisk together the all-purpose flour, cornstarch, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, cream together butter, white sugar and brown sugar using a stand or hand mixer. Mix over medium speed for about 1 minute. Add the egg and vanilla extract and continue mixing for another minute.
Slowly beat in the dry ingredients over low speed. Stir in the the oatmeal. I use a wooden spoon at this point instead of the mixer. Fold in the chocolate chips.
Using a cookie scoop (see note 1), portion out cookie dough on the parchment paper lined baking sheet. Place cookie dough about 4 inches apart. Bake at 350 for 13-15 minutes, or until the edges are set and golden and the center is still soft. Remove from the oven and cool on the baking sheet for 5 minutes before transferring to a wire cooling rack to cool completely.
Cookie scoop: For medium to large cookies, use a cookie scoop, which is a 2 ounce volume (equal to 4 tablespoons). For smaller cookies, use a large spoon to make cookie dough portions half the size (about 2 tablespoons) and bake for 9-11 minutes.
Make-ahead instructions: You can make the cookie dough up to 3 days in advance. Cover tightly and keep in the fridge until you are ready to use it.
Freezing: You can also freeze the cookie dough. Freeze all of the dough together in one container or make individual cookie dough balls. Let the cookie dough container thaw in the fridge until malleable. The frozen cookie dough balls only need to cook one minute longer in the oven.