Shortbread:

Preheat the oven to 350 degrees F.

Butter 2 (8-inch) square nonstick pans and coat with flour, tapping off excess. Place the flour, sugar and salt in a food processor and pulse once. Add butter and pulse until mixture resembles peas. Press the shortbread mixture into prepared pans and bake until golden brown around the edges, about 20 minutes. Remove from the oven and let cool completely.

Caramel Layer:

The easiest way is just to buy pre-made dulce de leche, places with a decent selection of Hispanic foods normally have it in a can at a very reasonable price. If you don’t have that, you can make your own: In a heavy-bottomed saucepan over medium-low heat, or in the top of a double boiler combine the condensed milk and 2 tablespoons of butter. Slowly bring the mixture to a boil, stirring continuously. Continue stirring over the heat until mixture becomes thick and amber in color, about 15 minutes. If you prefer, melting caramel candies with a little butter & salt would also work. Pour the dulche de leche/caramel over the cooked shortbread and spread evenly using an offset spatula (if you use dulche de leche from a can I’d advocate warming it up first, then adding a pinch of salt). Cool to room temperature.

Chocolate Topping:

In a glass bowl set over a saucepan of simmering water, melt the chocolate and butter. Once chocolate has melted, pour it over the cooled caramel layer. Cool at room temperature for about 10 minutes, sprinkle with fleur de sel (if desired) and then place in the refrigerator to cool completely, allowing chocolate to slightly harden, it’ll get a more matte look to the chocolate when ti’s firm. Cut into 2-inch squares and enjoy. To store, put it in an airtight container, at room temperature or keep in the fridge (recommended if your house is warm enough that these can be ultra-messy at room temperature).