Preheat your oven to 400°F. In a smaller baking dish, pour the sweetened condensed milk. If you’re using the cinnamon stick, toss it in now. Cover this smaller dish firmly with foil. Place it inside a larger baking dish and pour water into the larger dish, filling it halfway. This setup creates a water bath, ensuring your condensed milk cooks evenly and doesn’t scorch. Bake for 1 hour to 1 hour and 15 minutes, checking around the 45-minute mark to refill the water if needed.

Once time’s up, carefully remove the dish from the oven and let it cool for about 15 minutes. Unwrap the foil, discard the cinnamon stick, and transfer the rich dulce de leche into a mixing bowl. Whisk it until smooth. You can make this ahead of time; it keeps well in the fridge for up to two weeks. If it stiffens, place the container in warm water to bring it back to a spreadable consistency.

In a medium bowl, whisk together the sifted flour, corn starch, powdered sugar, cocoa powder, and salt. Set this dry mix aside for a moment. In a stand mixer (or a regular bowl with an electric mixer), beat the butter and vanilla together until they’re beautifully smooth. If you’re adding the egg yolk, mix it in next until just combined.

Gradually add the dry flour mixture to the butter mixture, being careful to mix slowly. You don’t want flour flying everywhere! Once combined, you’ll have a soft dough. Scoop the dough onto a piece of plastic wrap, form it into a ball, and flatten it into a thick disc. Wrap it tightly and pop it in the fridge for about an hour.

Once your dough has chilled, preheat your oven to 350°F. Take the dough out of the fridge and place it on a sheet of parchment paper. Place another sheet of parchment on top and roll it out gently until it’s about 1/4 to 1/2 inch thick. If the dough feels too cold, let it sit for about 10 minutes to soften slightly.

Using a 3-inch cookie cutter, cut out as many cookies as you can, making sure to keep them close together. Transfer the cookies to a lined baking sheet, spacing them about two inches apart. Pop them back into the refrigerator to chill for another 10 minutes so they don’t lose their shape while baking.

Before placing the cookies into the oven, score the tops gently with a fork, making lovely little lines. Bake for about 11 to 13 minutes. The cookies should feel firm to the touch but will have no color on the edges. Once done, allow them to cool on the baking sheet until they reach room temperature.

When the cookies are cool, flip half of them upside down on a clean surface. Fill a piping bag with the luscious manjar blanco and pipe a generous amount on each of the flipped cookies. If you don’t have a piping bag, you can spoon the filling onto the cookies. Gently top each filled cookie with another one, press lightly, and then roll the sides in sifted powdered sugar to finish.

Store your wonderful alfajores in an airtight container for about 3 to 5 days. Watch them disappear!