1. In a small bowl, whisk together flour, cornstarch, baking powder and salt. Set aside.

2. In a large bowl with a hand mixer or a stand mixer, beat butter until pale and fluffy, about 3 minutes. Add in both sugars, beat again for another 3-4 minutes until pale and fluffy. Add in egg yolks and beat until smooth.

3. On the lowest setting, add in flour mixture and stir until dough comes together. Chill dough for at least 3 hours (or up to overnight).

4. When ready to bake, preheat oven to 300 degrees and line two cookie sheets with parchment paper. Remove dough from fridge and place on lightly floured surface. Roll dough out to ½ inch thickness, then use a 2 ½ inch cookie cutter to cut out cookies. Gather dough scraps and repeat until all dough is used. Place cookies on prepared baking sheet, leaving ½ inch between each cookie.

5. Bake at 350 degrees for 12-15 minutes, or until edges just begin to brown and are set.

6. Let cool on cookie sheets for 10 minutes before removing to wire rack to cool completely.

7. Once cool, use dulce de leche to sandwich two cookies together. Decorate with melted chocolate, if desired.