Preheat the oven to 180°C then butter and line an 8-inch (or 9-inch) square pan. Set aside.
In a large bowl, using an electric handheld mixer, cream together the butter and sugar until light and fluffy.
Add in the flour and salt and mix until the dough comes together. Press into the pan until leveled.
Bake for roughly 32-35 minutes or until lightly browned on top. Set aside to cool completely.
For the Caramel Filling
Place the sweetened condensed milk and butter in a heavy-bottomed saucepan. Bring to a simmer over medium-low heat while whisking constantly.
Continue to simmer over low heat until the mixture thickens and turns into a light amber color, about 15-20 minutes. Remember to whisk all the time so the caramel won't stick to the bottom of the pot and burn.
Pour the thick golden caramel over the cooled shortbread and spread evenly. Allow to fully cool at room temperature for about 2-3 hours.
For the Chocolate Layer
Melt the chocolate and pour over the caramel. Smooth until leveled. Allow setting at room temperature for the chocolate to harden and cut into 16 squares using a warm knife.