Beat butter in the bowl of a stand mixer (or using an electric beater) for about 30 seconds.

Add sugars and beat until light and fluffy (pause to scrape down sides and bottom of bowl, if needed).

Add eggs, one at a time, beating until combined.

Stir in vanilla extract.

In a separate bowl, whisk together flour, cornstarch, baking soda, salt, and cinnamon.

Gradually add flour mixture to butter mixture until completely combined. Be sure to scrape the sides and bottom of the bowl so ingredients are well-mixed.

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.

Bake on 375F (190C) for 10-12 minutes (edges should be slightly browned, centers may still be slightly underbaked but shouldn't be raw).

Allow cookies to cool completely on cookie sheet before serving and enjoying.