Preheat the oven to 350°F. Line 2 cookie sheets with parchment paper.

In a medium bowl, whisk together flour, cornstarch, baking powder and salt; set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and powdered sugar until light and soft, about 3 minutes. Beat in the vanilla. Add the dry ingredients and beat at low speed, scraping the side of the bowl occasionally, until smooth. Roll the dough into teaspoon-size balls and place on the prepared baking sheets about 1 1/2 inches apart. Press each cookie with the back of a fork to flatten the cookies slightly. Bake the cookies, rotating the pans from top to bottom and front to back midway through, for about 15 minutes or until very lightly golden. Cool the cookies completely on a rack.

To assemble, Spread a layer of Nutella on the flat side of 1 cookie. Top with second cookie, forming a sandwich. Repeat with remaining filling and cookies.