Preheat the oven to 350°F (180°C). Line three baking sheets with parchment paper and set aside. In a large mixing bowl, whisk together the flour, cornstarch, powdered sugar, baking powder and salt until combined.
Flip half of the cookies upside down so that the browned side is facing up, and pipe or spread a generous amount of dulce de leche on top. Place a second cookie, browned side down, on top of the dulce de leche and gently squish together.