• In a medium bowl, sift the cornstarch, baking powder, and flour; set aside. In a stand mixer bowl fitted with a paddle attachment, cream the unsalted butter, salt, and confectioners' sugar at medium-high speed until light, smooth and creamy for about 3 minutes. Beat in pure vanilla extract and clear vanilla; you can also add the optional lemon zest at this step.
• Beat in the egg yolks one at a time, and mix briefly to incorporate until smooth, scraping down the sides of the bowl with a rubber spatula, as needed.
• Reduce the mixer speed to medium-low and gradually beat the flour mixture until the dough comes together; do not over-mix, or the cookies will turn out tough.
• Form the dough into two disks, wrap tightly in plastic wrap and refrigerate until firm, about 1 hour. (If you don't want to bake the alfajores right away, you can wrap the cookie dough in plastic wrap, store it in an airtight container and refrigerate it for up to three days, or freeze it for up to one month).
• Position racks in the upper and lower thirds of the oven and preheat to 350°F. Line 2 baking sheets with parchment paper; set aside.
• Take the dough out of the fridge and let it sit on the counter for a few minutes to soften slightly for easy rolling. Sandwich 1 disk of dough between 2 sheets of parchment and roll the dough ¼ inch thick.
• Cut out rounds using a 2 ½-inch cookie or biscuit cutter and put them on the prepared baking sheets about 1 inch apart (about 12 cookies per sheet). Repeat with the second disk of dough. (If the dough becomes too warm at any point, put it back into the fridge for a few minutes. Then, re-roll the remaining scraps and repeat the process).
• Bake the alfajores, rotating the baking sheets halfway through, until set, and the underside of the alfajores appears lightly golden brown but has not taken on any color on top, 9 to 10 minutes. Let the alfajores cool on the sheets for 3 minutes, then transfer to a wire rack to cool completely. The cookies can be stored at room temperature in an airtight container for up to 3 days.

How to Assemble the Alfajores

• Scoop the Dulce de Leche in a piping bag fitted with a plain tip. Pipe a generous amount of Dulce de Leche on one-half of the cookies and use the remaining cookies to top them, creating a sandwich by pressing gently until the caramel filling reaches the edges; you could use a butter knife if you prefer.
• Roll the edges in unsweetened shredded coconut. Alternatively, you can dust them with powdered sugar or dip them in melted chocolate. Then, enjoy the Alfajores with a cup of cold milk!