Step 1: Prepare Dry Ingredients
In a medium bowl, whisk together the cornstarch, all-purpose flour, baking powder, and salt. Set this mixture aside. If using lemon zest, add it to the dry ingredients and mix well to distribute evenly.

Step 2: Cream Butter and Sugar
In a large bowl or stand mixer fitted with the paddle attachment, beat the softened butter and powdered sugar together on medium speed for 3-4 minutes until light, fluffy, and pale in color. The mixture should look almost white and have increased in volume. Scrape down the sides of the bowl as needed.

Step 3: Add Egg Yolks and Vanilla
Add the egg yolks one at a time, beating well after each addition. Mix in the vanilla extract. The mixture should be smooth and creamy after this step.

Step 4: Combine Wet and Dry
With the mixer on low speed, gradually add the dry ingredient mixture to the butter mixture. Mix just until the dough comes together and no flour streaks remain. Do not overmix or the cookies will be tough. The dough will be soft and slightly crumbly but should hold together when pressed.

Step 5: Chill the Dough
Divide the dough into two portions and shape each into a flat disk. Wrap each disk tightly in plastic wrap and refrigerate for at least 1 hour, or up to 24 hours. This chilling step is crucial – it makes the dough easier to roll and helps the cookies hold their shape during baking.

Step 6: Preheat and Prepare
When ready to bake, preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats.

Step 7: Roll Out Dough
Working with one disk at a time (keep the other refrigerated), roll the dough out on a lightly floured surface to about ¼-inch thickness. The dough is delicate, so work gently and quickly. If the dough becomes too soft, return it to the refrigerator for 10-15 minutes.

Step 8: Cut Cookies
Using a 1.5 to 2-inch round cookie cutter, cut out circles from the dough. Gather scraps, re-roll once, and cut more cookies. Place cookies on prepared baking sheets about 1 inch apart (they don’t spread much).

Step 9: Bake
Bake for 10-12 minutes, just until the cookies are set but still pale. Do not let them brown – they should remain almost white with just barely golden edges. They will seem soft when they come out but will firm up as they cool.

Step 10: Cool Completely
Let cookies cool on the baking sheet for 5 minutes, then carefully transfer to a wire rack to cool completely. These cookies are extremely delicate when warm, so handle gently.

Step 11: Assemble Cookies
Once completely cool, pair up cookies of similar size. Spread about 1-2 teaspoons of dulce de leche on the flat side of one cookie (adjust amount based on cookie size). Top with the matching cookie, flat side down, and press gently to spread the filling to the edges.

Step 12: Add Coating (Optional)
If using coconut, place shredded coconut in a shallow bowl or plate. Roll the edges of each assembled cookie in the coconut, pressing gently so it adheres to the dulce de leche. Alternatively, dust the tops with powdered sugar using a fine-mesh sieve.

Step 13: Set and Serve
Let the assembled alfajores rest for about 30 minutes before serving to allow the dulce de leche to set slightly. This also lets the flavors meld beautifully.