Preheat the oven to 350ºF degrees. Line 2 baking sheets with parchment paper and set aside.
Combine the flour, baking soda, cornstarch, and salt; stir with a whisk. Set aside.
Combine melted butter and sugars in a large mixing bowl. Stir with a whisk until the mixture is smooth and free of clumps. Whisk in the egg, egg yolk, and vanilla.
Add the flour mixture and stir just until well blended. Fold in the chocolate toffee pieces and butterscotch chips.
Roll cookie dough into 1.5 tablespoons sized balls and place them 2 inches apart on the prepared baking sheets.
Bake cookies for 12 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 one week stored at room temperature in an airtight container.
The dough can be made ahead and stored in the refrigerator for up to 3 days. Bring the dough to room temperature and continue with step 6.
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