Preheat oven to 350°F (175°C). Line a large baking sheet with a Silpat baking mat or parchment paper and set it aside.

In a medium bowl, whisk together 2 1/4 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon cornstarch, 1/2 teaspoon salt, and 1/4 teaspoon ground cinnamon. Set the dry mixture aside.

In the bowl of an electric stand mixer (or a large bowl using a hand mixer), place 3/4 cup unsalted butter (room temperature) and 1 1/4 cups dark brown sugar. Beat on medium speed until the mixture is light and fluffy, about 3 minutes.

Turn off the mixer and scrape down the sides and bottom of the bowl with a spatula. Add 1 large egg and 1 teaspoon vanilla extract, then beat on medium speed until just combined.

With the mixer on low speed, add the dry ingredients to the wet ingredients in two additions, mixing only until just combined. Do not overmix.

Remove the bowl from the mixer and fold in 1 cup butterscotch chips with a spatula until evenly distributed.

Scoop tablespoons-size portions of dough (about 1 tablespoon each) and roll each portion into a ball. Roll each dough ball in the extra dark brown sugar (about 1/2 cup) until coated.

Place the coated dough balls on the prepared baking sheet, spacing them about 2 inches apart to allow for spreading.

Bake in the preheated oven for 10 to 12 minutes, or until the edges are lightly browned and the centers look set but still soft. Do not overbake.

Remove the baking sheet from the oven and let the cookies cool on the pan for 2 minutes. Transfer the cookies to a wire cooling rack and cool completely.

Store cooled cookies in an airtight container for up to 4 days.