White Chocolate Covered Pretzel Cookies

 cup (1.5 sticks) unsalted butter, softened to room temperature

packed dark brown sugar

 cup granulated sugar

 large egg, at room temperature

 teaspoons vanilla extract

 cups all-purpose flour

 teaspoons cornstarch

 teaspoon baking soda

 teaspoon salt

and 1/2 cups chopped chocolate covered pretzels with sprinkles (use small pretzel twists)