Preheat Oven:
Preheat oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy removal.
Make Shortbread Base:
In a large bowl, cream together butter and sugar until light and fluffy. Stir in flour and salt until a crumbly dough forms. Press evenly into the prepared pan. Bake for 18–20 minutes, until lightly golden. Remove from oven and cool slightly.
Prepare Caramel Layer:
In a medium saucepan over medium heat, combine sweetened condensed milk, butter, brown sugar, and golden syrup. Cook, stirring constantly, until thickened and golden, about 10–12 minutes. Remove from heat and stir in vanilla extract.
Assemble Caramel Layer:
Pour the caramel over the cooled shortbread base and spread evenly. Allow to set at room temperature for 30–40 minutes or refrigerate for faster setting.
Add Chocolate Topping:
Melt chocolate and butter together in a heatproof bowl over simmering water (double boiler) or in the microwave in 30-second intervals, stirring until smooth. Pour over the caramel layer and spread evenly.
Chill and Slice:
Refrigerate for at least 1 hour or until chocolate is set. Lift from pan using parchment overhang and cut into squares or rectangles.