Preheat the oven to 350 degrees F.; line cookie sheets with parchment or a silpat for easier cleanup.
Sift together the cornstarch, flour, baking powder and salt. Set aside.
Using hand held beaters or a stand mixer, cream the butter until smooth. Add the sugar gradually over the course of 2 minutes. Add the egg, then the yolks one at a time. Add the vanilla and lemon zest, stop and scrape the bowl as you go to ensure even mixing.
Add the cornstarch mixture in 4 parts, blending each addition thoroughly as you go. Drop the dough by the teaspoonful onto the prepared cookie sheets, leaving about 1 inch between each, as they will spread. Bake for 15 minutes. The color will remain light.
Allow cookies to cool completely; spread the flat side of half with the dulce-de-leche and press the flat side of another cookie against it immediately.
Store airtight for up to a week.
Yield about 2 1/2 dozen cookies.