1.  Sift flour, powdered sugar, cornstarch, and salt into a medium bowl.  Beat butter in a large bowl with an electric mixer on high until light and fluffy.  Add white sugar and vanilla; mix until combined. 

2.  Add flour mixture to butter mixture, a third at a time, mixing well after each addition until a dough forms.  Divide dough in half; roll each half into an 8" long log.  Wrap logs with plastic wrap and chill until firm, at least 2 hours.  

3.  Preheat oven to 350°.  Line 2 baking sheets with parchment paper. 

4.  Cut logs crosswise into 1/4" slices using a sharp, thin-blade knife.  Arrange cookies 1 1/2" apart on prepared baking sheets. 

5. Bake until edges are firm and just beginning to brown, 8-10 minutes.  Let cookies cool on baking sheets about 1 minute. Transfer to wire racks to cool completely, about 30 minutes. 

6.  Spread dulce de leche on the bottoms of half of the cookiesm using about 2 tsp each. Top with remaining cookies, bottom side down.  Lightly dust with additional powdered sugar.  Store cookies between layers of wax paper in an airtight container in the refrigerator up to 3 days,