In a medium bowl, whisk together 1 1/2 cups flour, 1/2 teaspoon baking soda, 1/2 teaspoon cornstarch, and 1/2 teaspoon salt until evenly combined; set aside.

In a large bowl, use an electric mixer to cream 1/2 cup softened unsalted butter and 1 cup brown sugar until light and fluffy, about 2–3 minutes.

Add 1 large egg and 2 teaspoons vanilla extract to the butter mixture and mix with the electric mixer until fully combined.

Add the dry flour mixture to the wet ingredients and mix with the electric mixer until a cohesive dough forms and no dry streaks remain.

Stir in 1 cup chocolate chips and 1 cup toffee pieces (plain or chocolate-covered) by hand or on low speed until just combined.

Cover the dough and refrigerate (chill) for 30 minutes.

About 10 minutes before the end of chilling, preheat the oven to 350°F and place a rack in the center position. Line a baking sheet with parchment paper or lightly spray it with cooking oil.

Remove the dough from the refrigerator and portion it into balls, placing them about 2 inches apart on the prepared baking sheet (about 6–8 cookies per pan).

Bake on the center rack for 10–12 minutes, until the edges are beginning to turn golden brown and the centers remain soft.

Transfer the cookies to a wire cooling rack and let cool until set. Enjoy!