Preheat the oven to 350ºF degrees. Line 2 baking sheets with parchment paper; set aside.
In a medium-sized bowl, combine the flour, cornstarch, baking soda, baking powder, and salt; stir with a whisk.
Beat the butter and sugar together in a large bowl using a stand mixer with a paddle attachment or a hand-held mixer on medium-low speed just until combined. Beat in the egg, egg yolk, and vanilla.
Add the flour mixture all at once and mix until just combined. Fold in the butterscotch chips.
Roll cookie dough into 1 ½ tablespoon-sized balls and place them 2 inches apart on the prepared baking sheets.
Bake one sheet of cookies at a time for 8-10 minutes or until lightly brown and just underdone.
Cool the cookies on the pan for 10 minutes. Remove from the pan and cool completely on a wire rack.
Notes
Make ahead tip
The cookies will keep for up to 1 week stored at room temperature in an airtight container.
Freeze baked cookies for up to 3 months. Thaw in the refrigerator overnight.
Freeze unbaked cookie dough balls for up to 3 months. Bring to room temperature and bake as directed.