1 jar (13 ounces) Nutella (you may not use the whole thing)
Preheat oven to 325°F. Line two baking sheets with parchment paper.
Combine the flour, cornstarch, baking powder, and salt in a medium bowl; set aside.
In a stand mixer fitted with the paddle attachment, cream the butter; once fluffy, add the sugar and beat until smooth. Beat in the egg yolks and vanilla extract. Gradually stir in the flour mixture, pausing to scrape down the sides of the bowl with each addition.
Pull a piece of dough, about 2 tablespoons, from the bowl. Form a 2-inch flat (but fairly thick) circle by hand. Spoon about 1 1/2 teaspoons of the filling on top of this dough-pancake. Close the dough over the filling to form a soft dome, making sure dough is covering the filling on all sides, and pinch the top to seal the cookie (it might resemble the top of a Hershey's Kiss; you can also flatten it with your hand, but make sure that the top of the cookie is sealed!).
Place cookies on the prepared sheets, 1 1/2 inches apart. Bake for 14-18 minutes, or until golden. Let cool on the pans. If desired, garnish with confectioners' sugar.