Whisk the flour, cornstarch, baking soda, and salt together in a medium bowl. Set aside.

In a large bowl using a handheld mixer or stand mixer with paddle attachment, beat the butter, brown sugar, and granulated sugar together on medium speed until combined and creamy, about 2 minutes. Beat in the egg and vanilla. Scrape down the sides and bottom of the bowl as needed.

On low speed, slowly beat the dry ingredients into the wet ingredients until combined. The cookie dough will be thick. Beat in the pretzels, butterscotch morsels, and chocolate chips on medium speed until combined. Cover dough tightly with aluminum foil or plastic wrap and chill for at least 1 hour and up to 3-4 days.

Preheat oven to 350°F (177°C). Line two large baking sheets with parchment paper or silicone baking mats. (Always recommended for cookies.) Set aside.

Roll into balls, about 1.5 Tablespoons of dough each (I like using this medium cookie scoop), and arrange 2-3 inches apart on prepared baking sheets. Bake for 11-12 minutes or until edges are slightly browned. The centers should still look soft. While the cookies are still warm, press additional morsels or chips into the top (if desired and only for looks!).

Cool cookies on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.

Cookies stay fresh covered at room temperature for up to 1 week.