Beat butter in a large bowl for about 30 seconds. Aim for a whipped consistency.
Add sugars and beat until light and fluffy, scraping down the sides of the bowl as needed.
Add eggs, one at a time, beating until combined.
Stir in vanilla extract.
In a separate bowl, sift together flour, cornstarch, baking soda, salt, and cinnamon.
Gradually add flour mixture to butter mixture until completely combined. I usually do half the dry mixture, beat until combined, other half.
Gradually stir in oats until completely combined. If using raisins or chocolate chips, stir them in at this point.
Cover dough with plastic wrap and place in the refrigerator to chill for at least 30 minutes and up to 3 days.
Preheat oven to 375F (190C) and prepare cookie sheets by lining with parchment paper.
Drop cookie dough by rounded 2 Tablespoon-sized balls onto parchment paper, spacing at least 2" apart. If desired, roll cookies between your palms to make a smooth ball, which will give you more uniform cookies.
Bake on 375F (190C) for 15-20 minutes (edges should be slightly browned, centers may still be slightly underbaked but shouldn't be raw, they'll bake completely as they cool).
Allow cookies to cool completely on cookie sheet before serving and enjoying.
I added tap water in steps 6 and 7, just to make sure it's not a solid lump of oatmeal. There isn't a whole lot of liquid in this recipe to begin with, and adding some helps. The batter will still be thicker than you expect, but not rock-solid.
