by Stacy Adimando from Cookiepedia
Cream the butter and sugar together for a minute or two, until they look light and fluffy.
In the meantime, sift the flour, cornstarch, baking powder, and salt into a bowl and set it aside.
Add in the egg and egg yolk one at a time, mixing after each addition. add vanilla extract and mix briefly. add in flour mixture and mix just until the dough starts to come together.
Working quickly, turn out the dough and use a little heat from your hands to make it a solid ball. pullout a large plastic wrap and flatten the dough on top to make a disk. double wrap it and refrigerate for 1 hour, until firm.
Line several cookie sheets with parchment paper or silicone mats. roll dough to 1/4 inch thick on a lightly floured surface. rotate the dough between rolls to make sure it’s not sticking to the surface.
Using a 2 inch fluted or round cutter, cut out cookies and carefully transfer them onto sheets, placing them about 1 inch apart.
Chill sheets again for about 15-20 minutes, until the dough is very firm. then bake cookies for 8-10 minutes, until tops of the cookies have just firmed and bottoms are starting to color slightly.
Cool the sheets on wire racks before assembling the sandwiches.
Drop, pipe or spread a teaspoon of dulce de leche into the center of each cookie, then top with another. sift powdered sugar over the assembled sandwiches.
Note
You can make your own dulce de leche by placing several cans in a crock pot, covering them with water and cooking for 6 hours over low heat. This is what I did and used for the recipe.