Preheat the Oven:
Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.

Prepare the Dough:
In a large mixing bowl, cream together the softened butter and sugar until light and fluffy. This should take about 3-5 minutes.

Add Eggs and Vanilla:
Beat in the egg yolks, whole egg, and vanilla extract until well combined.

Mix Dry Ingredients:
In another bowl, whisk together the flour, cornstarch, baking powder, and salt. Gradually add this dry mixture to the wet ingredients, mixing until just combined.

Chill the Dough:
Wrap the dough in plastic wrap and refrigerate for about 1 hour. This will make it easier to handle.

Roll and Cut the Cookies:
Once chilled, roll out the dough on a lightly floured surface to about 1/8-inch thickness. Use a round cookie cutter (about 2 inches in diameter) to cut out circles and place them on the prepared baking sheet.

Bake:
Bake the cookies for about 10-12 minutes, or until the edges are lightly golden. Allow them to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.

Assemble:
Once the cookies are completely cooled, spread a generous layer of dulce de leche on the flat side of one cookie, and sandwich it with another cookie.

Coat the Alfajores:
If desired, roll the edges of the alfajores in grated coconut or dust them with powdered sugar for an added touch.

Serve:
Enjoy your alfajores with a cup of coffee or tea, or store them in an airtight container for a delicious treat later on!