Crust: Heat oven to 350. Line bottom and sides of 13x 9 pan with foil. Combine flour, sugar, and salt in medium bowl. Add melted butter and stir until flour is evenly moistened. Crumble dough evenly over bottom of pan. Using your fingers, press and smooth dough into even thickness. Using fork, pierce dough at 1-inch intervals. Bake until light golden brown and firm to touch, 25 to 30 minutes. Using metal spatula, press on entire surface of warm crust to compress (makes cutting finished bars easier). Cool at least 20 minutes.

Caramel Filling: Stir ingredients together in large, heavy-bottomed saucepan. Cook over medium heat, stirring frequently, until it registers between 236 and 239 degrees, 16 to 20 minutes. Pour over crust, and spread evenly. Cool completely, about 90 minutes.

Chocolate: Microwave 6 ounces chopped chocolate at 50 percent power, stirring every 15 seconds, until fully melted but not much warmer than body temperature, 1 to 2 minutes. Add 2 ounces grated chocolate, and stir until melted, returning to microwave for no more than 5 seconds at a time to complete melting if necessary. Spread evenly over surface of filling. Refrigerate until chocolate is just set, about 10 minutes.

Using foil, remove shortbread from pan and transfer to cutting board. Using serrated knife and gentle sawing motion, cut shortbread in half crosswise to create two 6 1/2- by 9-inch rectangles. Cut each rectangle in half to make four 3 1/2 - by 9-inch strips. Cut each strip crosswise into equal 10 pieces. (Shortbread can be stored at room temperature, between layers of parchment, for up to one week.)

Notes: To ensure that the caramel filling has the perfect texture, monitor the temperature with an instant read thermometer. We prefer Ghirardelli 60% Cacao Bittersweet Baking Bar for this recipe. Grating a portion of the chocolate is important for getting the chocolate to set properly. The small holes on a box grater work well for this task. Stir often while melting the chocolate and monitor the temperature by holding the bowl in the palm of your hand; overheating the chocolate will cause it to set poorly.