In the medium bowl, sift together the cornstarch, flour, and baking powder. In the bowl of the mixer, beat together the sugar and butter until fluffy, about 2 minutes. Add the cognac and zest and beat. Add the egg yolks one at a time, beating after each until all the eggs are thoroughly mixed in. Add the dry ingredients and mix on low until fully incorporated.
Transfer the dough to a lightly floured surface and knead briefly, then divide it into 3 pieces. Working with 1 dough piece at a time, roll out the dough to 1/4″ thickness. Cut out the cookies with the round cutter and transfer the cut cookies to the prepared baking pans, spacing them 1 inch apart. Reroll the scraps and repeat until all the dough has been cut.
Bake the cookies in the preheated oven until golden, about 12–15 minutes. Let cool. Flip half the cookies over and top each with 1 heaping teaspoon of dulce de leche. Top with remaining cookies. Store in a tin for up to 3 days.