6 ounces dark chocolate (60% cacao), coarsely chopped
1 teaspoon light corn syrup
1/2 cup (1 stick) unsalted butter
Sea salt, for finishing
For the shortbread:
Preheat oven to 350° Fahrenheit. Butter a 9″x13″ baking dish and set aside.
Beat the sugar and butter together until blended. Add the flour and the salt and beat until well combined. Add the egg yolk and beat until just combined. Dough will be crumbly.
Bring the dough together and turn it out into the prepared baking dish. Press into the pan in an even layer. Use the back of a spoon to even it out. Prick all over with a fork.
Bake in the middle rack of the oven for 20 to 22 minutes, until golden brown. Transfer to a wire rack and let cool completely.
For the caramel filling:
Put the sweetened condensed milk in a very large microwave-safe bowl. Cook on 50% power (medium) for 4 minutes, stirring briskly halfway through, until smooth. Watch carefully as it can bubble and boil over. Decrease the power to 30% (medium-low) for 12-18 minutes, stirring every 2 minutes, until it is very thick and bubbly, and caramel colored.
Pour the caramel filling over the cooled shortbread and place the pan in the refrigerator until cool, at least an hour.
For the chocolate glaze:
Melt the chocolate, corn syrup, and butter in the microwave until completely smooth.
Pour the mixture over the chilled caramel layer and spread it into an even layer. Let the layer set slightly in the fridge (about 2 minutes or so) and then sprinkle with sea salt. Chill in the refrigerator for at least 1 hour, or until it hardens.
Remove the pan from the refrigerator 30 minutes before serving so as not to crack the chocolate layer. Cut into squares and serve. They can be stored in the refrigerator, tightly covered, for up to 4 days.