In a medium bowl, sift together flour, cornstarch, baking powder, baking soda, and salt. Set aside.

In a small bowl, rub the lemon zest and granulated sugar together with your fingers.

In a mixer bowl fitted with the paddle attachment, beat the lemon zest-sugar mixture with butter until light and fluffy. Beat in egg yolks one at a time, scraping down the bowl between and after additions. Add vanilla extract and mix just until combined. Reduce speed to low and add the flour mixture, beating just until combined.

When ready to roll out cookies, let dough rest at room temperature for 5 to 10 minutes before rolling. On a lightly floured surface roll the dough out to 1/4-inch thickness. Cut out into rounds using a 2-inch cookie cutter and place the cookies on baking sheets lined with parchment paper. (Fluted cookie cutters are traditional but round is fine.)

Gather scraps and re-roll to get 24 to 28 cookie halves. Chill cookies in the freezer for 10 minutes before baking.

Preheat oven to 350°F. Bake for 10-12 minutes, or until cookies just start to appear golden brown at the edges. Cool cookies in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Spread about a teaspoon of dulce de leche on the bottom half of the cookies. Sandwich with remaining top halves, pressing slightly and twisting to evenly distribute filling. Dust the tops with confectioners sugar.