In a stand mixer, fitted with the paddle attachment, combine the room temperature butter and powdered sugar. Beat on medium speed for 2-3 minutes until light and fluffy. 

Add the egg yolks, one at a time, until thoroughly combined. Lower the speed of the mixer, and add the lemon zest, corn starch, flour, baking soda, baking powder, and salt. Mix JUST until a loose dough has formed, about 30 seconds.

Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper, set aside. Pour the dough out onto a well-floured work surface, and use your fingertips to shape it into a ball. Roll the dough out using a rolling pin to 1/4" thickness. Use a 3-4 centimeter round cutter to cut out the cookies. You can re-roll the dough scraps, and cut more cookies.

Place the dough rounds onto the prepared baking sheet, and bake for 7-8 minutes ONLY. The should be puffy, and pale. Let cool for 10-15 minutes. Spoon a generous teaspoon of dulce de leche onto 1/2 of the cookies, sprinkle with a bit of sea salt, and sandwich the top cookie on. Serve at room temperature. OPTIONAL: roll the edges of the cookie in the unsweetened coconut flakes. Store in an airtight container in the fridge. Allow to come to room temperature before serving.