For the shortbread base:

1. With a wooden spoon or electric hand mixer on low, cream butter with sugar until smooth.
2. Add salt and flour in two parts, combining well. Do not overmix. It will be crumbly.
3. Line an 8-inch square pan with aluminum foil or parchment paper and pat the dough evenly on the bottom. Lightly flour the tips of your fingers if needed to do this. Be sure all surface is covered as evenly as possible.
4. Prick the bottom with a fork and freeze for 30 minutes.
5. Preheat the oven to 350ºF / 180ºC.
6. Bake for about 20 minutes, until it’s dry and fully baked. Reserve.

For the caramel:

1. Combine the butter, brown sugar, salt, vanilla, sweetened condensed milk and syrup.
2. Place over medium heat and whisk until melted and combined, then continue whisking over the heat while the caramel bubbles and turns a darker, richer color. If you use a candy thermometer it needs to reach 225°F. If you don’t, you'll see it starts to thicken and pull away from the edge, 5-6 minutes; your caramel is ready.
3. Spread evenly over the cooled shortbread crust.
4. Refrigerate until the caramel sets.

For the chocolate topping:

1. Place chopped chocolate in a medium bowl. 
2. Heat cream in a small saucepan until it's about to break into a boil. 
3. Pour over the chocolate, let stand 30 seconds and whisk until smooth and shiny. 
4. Spread over the cold caramel, covering it completely. 
5. Leave at room temperature until it cools down, and then refrigerate, covered, until it solidifies.
6. When ready to serve, remove from the pan carefully, cut into squares and serve. Use a kitchen knife.