STEP ONESTIR together flour, cornstarch, baking powder, baking soda and salt in medium bowl until blended.
STEP TWOBEAT shortening and sugar in large bowl with electric mixer on medium speed until fluffy. Blend in egg yolks. Add water, lemon peel and vanilla, beating until blended. Gradually add flour mixture, scraping sides of bowl after each addition. Cover and chill 30 minutes.
STEP THREEHEAT oven to 325ºF. Coat baking sheets with no-stick cooking spray. Roll out half of dough to 1/4-inch thickness on lightly floured surface. Cut into 2-inch rounds. Place 2 inches apart on prepared baking sheets. Bake 9 minutes or until edges just begin to turn light brown. Cool 10 minutes. Remove to wire rack to cool completely.
STEP FOURTURN half the cookies flat-side up. Spread about one tablespoon dulce de leche on flat sides of cookies. Top with remaining cookies, flat-side down, to form sandwich cookies. Sprinkle with powdered sugar before serving, if desired.