How to Make It
Prepare the Crust: Preheat oven to 350°F. Pulse flour, powdered sugar, pecans, and salt in a food processor until thoroughly combined and pecans are finely chopped, 8 to 10 pulses. Add butter, and pulse until mixture resembles coarse meal, 8 or 9 pulses. Line bottom and sides of a 13- x 9-inch pan with aluminum foil, allowing 2 to 3 inches to extend over sides; lightly grease foil with cooking spray. Press mixture evenly and firmly into bottom of prepared pan.
Bake in preheated oven until lightly browned, 18 to 22 minutes. Cool completely on a wire rack, about 30 minutes.
Prepare the Caramel Layer: Stir together condensed milk, brown sugar, butter, cream, cane syrup, and salt in a large heavy saucepan. Cook over medium, stirring constantly, until a candy thermometer registers between 236°F and 239°F, about 20 minutes. Remove from heat, and stir in vanilla. Immediately pour over cooled crust, and spread to even thickness using a small offset spatula. Cool completely, about 2 hours.
Prepare the Chocolate Layer: Place chopped chocolate in a medium-size microwavable bowl. Microwave on MEDIUM power (50%) until melted and smooth, about 2 minutes, stirring every 30 seconds. Spread chocolate evenly over cooled caramel layer. Let stand 5 minutes, and, if desired, sprinkle with flaky sea salt. Chill shortbread until chocolate is just set, about 10 minutes.
Lift shortbread out of pan, using foil overhang as handles, and transfer to a cutting board. Cut shortbread in half crosswise to create 2 (6 1/2- x 9-inch) rectangles. Cut each rectangle in half to form 4 (3 1/4- x 9-inch) rectangles. Cut each strip crosswise into 8 equal pieces. Store shortbread between layers of parchment paper or wax paper in an airtight container at room temperature for up to 1 week.