Preheat oven to 325 degrees. Line an 8-inch by 8-inch baking pan with parchment, allowing two sides to have a 2-inch overhang. Spray with non-stick spray. Set aside.
In the bowl of a standing mixer fit with the paddle attachment on medium, or with an electric mixer by hand, cream together butter and sugar until fluffy and light, about 2 minutes.
Add flour and salt, and mix on low speed until combined and has the consistency of course sand, about 2 minutes. Transfer mixture to prepared pan and press into an even layer. Bake until light golden brown and dry, about 30 to 35 minutes. Cool on a cooling rack.
To make the caramel:
As the shortbread cools, combine sweetened condensed milk, butter, syrup, and salt in a medium sauce pan over medium heat. Stir continuously until the mixture is thickened and light butterscotch-colored, about 10 to 20 minutes. (Mixture should read 220 to 225 degrees on a digital thermometer.) Adjust heat as necessary and stir constantly to prevent bottom from scorching.
Remove pan from heat and stir in vanilla. Immediately pour over the shortbread crust, smoothing it into an even layer.
To make the chocolate topping:
In a small, microwave-safe bowl, heat chocolate.Stir carefully and continue to microwave in 30-second increments until smooth, stirring after each increment.
Pour over caramel filling and spread evenly. Chill until chocolate is just set, about 30 minutes.
Cut parchment away from sides of pan and using parchment overhang, carefully remove from pan. Using a sharp knife, cut into 16 squares.
Corn syrup: Traditional shortbread is made with Lyle's Golden Syrup. It is located in the baking aisle near the syrups of well-stocked grocery stores or you can buy it online.
Yield: This recipe makes 16 bars approximately 2 inches by 2 inches in size.
Storage: Store bars covered in the refrigerator for up to one week. Bring to room temperature before serving.
Freezer: Wrap bars well individually in plastic wrap and then foil. Label, date, and freeze for up to 3 months. Thaw at room temperature before serving.