Preheat oven to 350 degrees Fahrenheit. Line a 9 x 14-inch baking pan with parchment paper, leaving an overhang on the sides.

In the bowl of a standing mixer, add the butter and mix on medium speed until the butter is well creamed. Add the sugar and mix for another 30 seconds. Scrape down the sides, then add the egg yolk and mix for another minute.

With the mixer running at low speed, add the flour 1/2 cup at a time, pausing in the middle to add the salt. Mix only until just combined - the mixture will be crumbly.

Pour the mixture into the prepared pan, and using your hands, press the dough evenly into the bottom of the pan (not up the sides). Bake at 350 for 15 to 18 minutes, or until the edges begin to turn a light golden brown. Remove from oven and let cool completely.

To prepare the caramel layer, combine the condensed milk, butter, brown sugar, and corn syrup in a large saucepan. While whisking constantly, bring the mixture to a boil over medium heat. Reduce temperature to medium-low heat to maintain a simmer and continue whisking until the caramel is a deep golden color and reaches a temperature between 220 and 225 degrees Fahrenheit (use a candy thermometer or instant-read thermometer to check). Remove immediately from heat and pour evenly over the prepared shortbread (use a spatula to spread if necessary.)

Allow the two layers to cool in the fridge for at least one hour before topping with chocolate.

To make the chocolate layer, combine the heavy cream and chocolate chips in a double boiler over low heat. Stir just until the mixture is completely smooth, then remove immediately from the heat.

Let the chocolate rest for 5 minutes, then spread it smoothly over the top of the caramel layer and sprinkle with salt flakes.

Chill in the fridge for at least 30 minutes, but ideally overnight.

To cut, remove from pan using the parchment paper overhang. Let it rest on a cutting board at room temperature for 30 minutes. Then, using a long, sharp, flat knife, gently score the chocolate layer before pressing through the remaining two layers. Cut slowly and gently for clean layers. Enjoy!