In a medium bowl, sift together the all-purpose flour, cornstarch, baking powder, and baking soda. Set aside.

In a large mixing bowl, cream the softened butter and sugar together until light and fluffy, about 2-3 minutes.

Add the egg yolks one at a time, mixing well after each addition. Stir in the vanilla extract and lemon zest.

Gradually add the dry ingredients to the wet mixture, mixing until a soft dough forms. Do not overwork the dough.

Wrap the dough in plastic wrap and refrigerate for at least 30 minutes to firm up.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

On a lightly floured surface, roll out the dough to about 1/4 inch thickness. Use a 2-inch round cookie cutter to cut out circles.

Place the cookies on the prepared baking sheet, spacing them about 1 inch apart.

Bake for 10-12 minutes. The cookies should be set but remain pale; do not let them brown.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

To assemble, spread a generous tablespoon of dulce de leche onto the flat side of one cookie and sandwich it with another.

Squeeze gently so the filling reaches the edges, then roll the sides in the desiccated coconut to coat the exposed dulce de leche.