Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper to prevent sticking.
In a medium bowl, whisk together the all-purpose flour, baking soda, cornstarch, salt, and ground cinnamon. Set this mixture aside.
In a large mixing bowl, use an electric mixer to cream the unsalted butter and dark brown sugar together until light and fluffy, about 2-3 minutes.
Beat in the large egg and vanilla extract until well combined.
Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined. Be careful not to overmix!
Gently fold in the butterscotch chips until evenly distributed throughout the dough.
Using a cookie scoop, portion out the dough and roll it into balls. Roll each ball in the extra dark brown sugar before placing it on the prepared baking sheet.
Bake for 10-12 minutes, or until the edges are lightly golden and the centers look slightly underbaked. They will continue to cook as they cool.
Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a cooling rack. Enjoy your delicious Brown Sugar Butterscotch Cookies warm or at room temperature!