Combine butter and sugars in a large bowl and stir until well-combined.

Add eggs and vanilla extract, stir well.

In a separate, medium-sized bowl, whisk together flour, cornstarch, baking soda, salt, and cinnamon (if using).

Gradually stir dry ingredients into wet until completely incorporated.

Add oats, stir until combined. Finally, stir in 1 ½ cups butterscotch chips.

Cover bowl with plastic wrap and transfer to refrigerator to chill for at least one hour.

Once dough has chilled, preheat oven to 350F (175C).

Drop cookie dough by 2 Tablespoon-sized scoops onto parchment paper lined baking sheets, spacing cookies at least 2” apart.

Bake cookies in 350F (175C) preheated oven for 12-13 minutes or until edges are beginning to look golden brown. Centers may look slightly underdone when cookies are finished baking, they will cook completely as they cool on the baking sheet (don’t over-bake or the cookies will be dry).

If desired, press additional butterscotch chips into the surface of the cookies within 2 minutes of them coming out of the oven.