Preheat oven to 350 degrees F. Line a 13x9x2-inch baking pan with heavy foil, extending foil over the edges of the pan. Grease foil; set pan aside. In a large mixing bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Add sugar and vanilla. Beat until well combined, scraping bowl occasionally. Beat in flour. Stir in crushed crackers. Spread mixture evenly into the prepared pan. Bake in the preheated oven for about 15 minutes or until lightly browned.

Meanwhile, in a heavy medium saucepan, combine unwrapped caramels and evaporated milk. Heat and stir over medium-low heat until melted and smooth. Pour hot caramel mixture evenly over hot baked crust. Bake about 8 minutes more or until top is bubbly.

In a medium microwave-safe bowl, combine chocolate pieces and shortening. Microwave, uncovered, at 50 percent power (medium) for 1-1/2 to 2 minutes or until melted and smooth, stirring once halfway through cooking and at the end of cooking. Pour melted chocolate over hot caramel layer, gently spreading chocolate evenly over caramel layer. Cool completely in pan on a wire rack. Using the edges of the foil, lift the uncut bars out of the pan. Cut into bars. Makes 24 bars.

Place bars in an airtight container; cover. Store at room temperature for up to 1 week or freeze for up to 3 months.