For Crust: MIX flour, butter, sugar and salt in large bowl with mixer at medium speed until mixture resembles coarse crumbs; press firmly into greased 13x9-inch baking pan.
BAKE 15 minutes at 350°F or until golden brown. Sprinkle chocolate chips over hot crust; let stand 5 minutes or until shiny and soft. 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 tightly covered container at room temperature.