Preheat oven to 350°F. Spray a 13x9x2-inch baking pan with cooking spray.
For Crust: Mix flour, butter, sugar and salt in large bowl with mixer at medium speed until mixture resembles coarse crumbs; press firmly into prepared pan.
Bake 15 minutes or until golden brown. Sprinkle chocolatechips over hot crust; let stand 5 minutes or until shiny andsoft. Spread chocolate evenly; set aside.
For Caramel: Cook butter, brown sugar and corn syrup in a heavy 2-quart saucepan to boiling, stirring frequently. Boil for 4 minutes without stirring. Add vanilla extract and mix well.
Pour caramel over chocolate-covered crust; spread evenly. Sprinkle with walnuts. Place chocolate chips and shortening in a small microwaveable bowl and microwave on HIGH (100%) for 30 seconds or until melted. Drizzle melted chocolate over bars.
Chill 1 hour to set; let stand at room temperature until softened. Cut into 2 x 1-inch bars. Store in a tightly covered container at room temperature.