Heat oven to 350 degrees F. Line a 13 x 9 x 2-inch baking pan with nonstick foil; overhang short ends slightly.
Stir the graham cracker crumbs, sugar, salt and butter in a bowl until evenly moistened. Transfer crumb mixture to prepared baking pan; press crumb mixture evenly over the bottom of the pan. Refrigerate 30 minutes.
Bake the crust at 350 degrees F. for 17 minutes or until lightly browned. Remove to a rack and cool completely.
Stir together cornstarch and milk. Bring condensed milk, butter and corn syrup to a boil over medium-high heat in a small saucepan, stirring constantly to prevent burning. Whisk in cornstarch mixture; cook 2 minutes, whisking, or until thickened. Pour over crust, spreading level. Bake at 350 degrees F. for 8 minutes or until topping starts to bubble. Remove to a rack; refrigerate for 2 hours or until cool.
Microwave chocolate and shortening for 1 minute or until melted, stirring halfway. Stir until smooth. Spread evenly over filling and refrigerate for at least 45 minutes or until set. Use foil to lift bar from pan to a cutting board. Cut into 16 bars.