Preheat oven to 350°F. Line 2 baking sheets with parchment paper.
In a large bowl, beat butter, brown sugar, and granulated sugar until light and creamy, about 2 to 3 minutes.
Beat in the egg and vanilla until smooth, scraping down the bowl.
In a separate bowl, whisk together flour, cornstarch, baking soda, and salt.
Add dry ingredients to the wet ingredients and mix on low just until combined. Do not overmix.
Fold in butterscotch chips, toffee bits, and pecans if using.
Scoop dough into 1.5 tablespoon portions and place 2 inches apart on prepared sheets. For thicker cookies, chill the scooped dough for 20 minutes.
Bake 9 to 10 minutes until edges are set and centers look slightly underbaked. Remove from the oven and let cookies rest on the sheet for 5 minutes.
Transfer to a wire rack to cool completely. Enjoy warm or at room temperature.
Notes
For bakery-style crinkles, gently tap the baking sheet once on the counter right after baking. Store cookies airtight at room temperature for up to 4 days or freeze baked cookies for up to 2 months.