In a large bowl, sift together the flour, cornstarch, baking powder, baking soda and kosher salt. Set aside
In the bowl of an electric mixer, beat sugar and butter until light and fluffy, about 3 minutes.
Add yolks and beat to combine.
Add the vanilla extract, water and lemon zest beat until just combined.
Add flour mixture, and mix on low speed until the dough just comes together.
Wrap the dough in plastic and chill until firm, at least 2 hours.
Heat oven 350 degrees. Line baking sheets with parchment paper.
Remove dough from plastic wrap and place on a lightly floured surface, flour a rolling pin and roll out to about ¼ inch thick. Cut the dough with a 2 inch cookie cutter.
Place on prepared sheet pans, then bake until firm and edges are pale golden and the middle of the cookie is firm, about 9 minutes. Transfer baking pan to a rack and allow to cool completely before removing from baking sheet.
Spread 1 tablespoon of dulce de leche onto the back of half the cookies. Place a second cookie on top and gently press to create a sandwich and repeat. Dust generously with powdered sugar before serving.
Roll sides in coconut and dust generously with powdered sugar before serving.