Traditional alfajores are a South American dessert made with tender shortbread cookies and a gooey dulce de leche center.
In a medium bowl, sift together the flour, powdered sugar, and cornstarch; set aside.
Using an electric mixer on medium speed, beat the butter until light and fluffy, about 2 minutes.
Add the egg yolk, vanilla extract, and salt, and mix until combined and smooth, scraping the sides of the bowl as needed.
Reduce the mixer speed to low and gradually add the flour mixture, mixing until the dough just comes together. This may take a few minutes, as the mixture will go from looking sandy to seeing small clumps, and then it will eventually come together into a dough.
Separate the dough in half and work with one piece at a time. Roll the dough ¼-inch thick on a well-floured surface. Using a plain round or fluted round 2-inch cutter, cut out the cookies and place them on the prepared baking sheets, leaving at least 1 inch of space between cookies. Reroll any scraps and continue cutting rounds.
Bake until slightly firm and set, 11 to 14 minutes. Allow to rest for 5 minutes on the baking sheet, then transfer to a wire rack to cool completely.
Assemble the Alfajores: Spread about 1½ teaspoons of dulce de leche on the bottom of half of the cookies, then top with the remaining cookies. Dust with powdered sugar before serving. You can also roll the sides in shredded coconut, if desired.