Preheat the oven to 350°F. Line a 9×13″ pan with parchment paper.
Shortbread Crust
In a bowl, combine the flour, butter, and sugar with a mixer or pastry cutter, until crumbly. Press the mixture firmly into the bottom of the prepared pan.
Bake for 12 minutes, or until it looks slightly puffed and dry. Remove from the oven and let cool while you prepare the filling.
Caramel Layer
In a large saucepan, combine the butter, brown sugar, condensed milk, and corn syrup. Heat over medium-high heat, watching carefully and stirring, until it comes to a boil.
Reduce the heat to medium and boil for 5 minutes, stirring continuously.
Stir in the salt and pour over the crust. Let it chill completely in the refrigerator before adding the chocolate, about 1 hour.
Chocolate Ganache
In a small saucepan, combine the chocolate and the cream. Cook over low heat, stirring continuously, until melted. Pour over the chilled caramel layer and spread evenly. Sprinkle with flaky sea salt, if using.
Let chill until the chocolate layer is set before slicing.