Line two baking sheets with parchment paper. Sift together the cornstarch, flour, baking powder and baking soda. Set aside.

Cream together the butter and sugar until pale and fluffy. Add the yolks, and mix well to blend.

Add the sifted dry ingredients, and gently mix to blend. Do not overwork the dough.

Divide the dough into two pieces. Lightly dust a clean work surface with flour. Roll out one of the pieces of dough to ¼ centimeter thick. Use a cookie cutter to cut out circles (I like them 4 centimeters in diameter), and place them about 1 inch apart on the prepared baking sheets.

When you have filled a baking sheet, preheat the oven to 350 degrees F, and place the baking sheet in the refrigerator to chill. Continue rolling and cutting out the dough, and chill the baking sheets as you fill them.

Bake the cookies, one sheet at a time, for 12 to 14 minutes, rotating the pan after six minutes, until the cookies feel firm to the touch but have not taken on any color.

Let the cookies cool completely, then put the dulce de leche in a piping bag fitted with a plain tip. Finely crush four of the cookies, and put the crumbs in a small bowl. Turn half of the remaining cookies upside-down and pipe on a generous “kiss” of dulce de leche. Top with another cookie, and very gently squeeze until the filling is even. Very gently press the sides of each filled cookie into the crumbs.