In a medium sized bowl, whisk together flour, cornstarch, salt, baking powder and baking soda and put aside.
In the bowl of your mixer, add butter and both granulated and brown sugars and cream together for 3-4 minutes on high until light and fluffy.
Next add in eggs one at a time beating until incorporated. Then add in pure vanilla extract and beat until smooth.
Turn down your mixer to low and add in flour mixture in three intervals beating after each addition until well-combined. Lastly, stir in chocolate chips and butterscotch chips.
Place dough in the refrigerator for at least 1 hour. Two hours is even better.
Preheat oven to 350 degrees.
Spray a cast iron or deep dish skillet with non-stick baking spray.
Pat all of cookie dough into the pan then bake for 20-35 minutes depending on how well done you want the cookie to be.
Cookie may seem underdone but let it cool until warm to begin setting then serve.