WHISK flour and cornstarch together in a medium bowl. Set aside. Beat butter in a large bowl with an electric mixer at medium speed until fluffy. Add sugar. Beat until light and fluffy, scraping down the sides of the bowl as needed. Add vanilla and beat until well blended. Add flour mixture a little at a time and beat on low speed scraping sides as needed until blended.
TURN dough onto plastic wrap and form into 2 disks. Wrap in plastic wrap. Refrigerate 20 minutes.
HEAT oven to 325º F. Spray a metal baking sheet lightly with cooking spray. Working with one disk at a time, roll dough on lightly floured surface to 3/8 inch thickness. Cut hearts with a 4-inch cutter. Use 1 1/2-inch heart cutter to cut smaller heart in upper right portion of half the cookies. Place cookies one inch apart on prepared baking sheet. Bake for 15 to 17 minutes until edges are golden. Do not allow shortbread to brown. Cool 2 minutes. Allow to cool completely on wire rack.
MELT chocolate chips in microwave-safe bowl in microwave for 1 minute. Stir. Return to microwave for 30 second increments stirring in between and rotating the bowl if necessary until completely melted. Add shortening. Stir until blended. Dip left half of each cookie in melted chocolate mixture. Cool on wax paper in refrigerator until set.
REMOVE from refrigerator. Allow to come to room temperature. Cover left side of cookie with paper towel. Sprinkle right side with powdered sugar avoiding heart cut-out as much as possible. Store in an airtight container for 5 to 7 days.
Dough may be cut with a 2-inch round or shaped cutter to make 72 single cookies, 36 sandwich cookies.