Preheat oven to 350 F. In the bowl of a stand mixer, cream together the butter, sugar and vanilla until combined. Add the flour and sprinkle in the salt. Mix just until the dough forms a ball.
Pat the mixture into the bottom of a greased 9 x 13 inch baking pan. Bake for 15-17 minutes or until set and lightly browned. Let cool completely
Before starting to make the caramel, have all the ingredients measured and ready. Dump the sugar in the bottom of a heavy saucepan. Over medium high heat, melt the sugar stirring or whisking constantly. It may clump as it melts, but that is okay. Keep stirring. Stir until the sugar is melted and a deep caramel color.
As soon as it is the right color, add the butter and whisk until melted. Remove from the heat. Pour the heavy cream in very slowly whisking as you go. Whisk until smooth. Let cool for about 5 minutes and then pour onto the cooled shortbread. Let cool slightly.
To make the chocolate layer, place the chocolate chips and shortening in a microwave proof bowl. Melt in the microwave in 15 second intervals, stirring between each. When the chocolate is smooth, pour onto the cooled caramel and spread evenly.
Let set completely before cutting.