Step 1: Prepare the Shortbread Base

1. Preheat the oven: Set your oven to 350°F (175°C). Line a 9×9-inch baking pan with parchment paper, leaving a slight overhang so that you can easily remove the cookies once they are done.
2. Mix the shortbread dough: In a medium bowl, cream the softened butter and granulated sugar together using an electric mixer. Beat for about 2-3 minutes, until the mixture is light and fluffy. Gradually add the flour and salt, mixing until the dough comes together. The dough should be slightly crumbly but press together easily when squeezed.
3. Press the dough into the pan: Pour the dough into the prepared baking pan and gently press it into an even layer using your fingers or the back of a spoon. Ensure that the dough is compact and evenly distributed across the base.
4. Bake the shortbread: Place the pan in the oven and bake for 15-20 minutes or until the shortbread is lightly golden at the edges. Once baked, remove the pan from the oven and let the shortbread cool while you prepare the caramel filling.

Step 2: Make the Caramel Filling

1. Melt the butter: In a medium saucepan, melt the 1/2 cup butter over medium heat.
2. Add the sugar and syrup: Once the butter is melted, add the brown sugar and golden syrup (or corn syrup). Stir constantly until the sugar dissolves and the mixture starts to bubble.
3. Cook the caramel: Allow the mixture to cook for 4-5 minutes, stirring frequently. It should thicken slightly and become smooth. If you have a candy thermometer, aim for a temperature of around 240°F (115°C), which is the soft-ball stage.
4. Add the cream, vanilla, and salt: Remove the saucepan from the heat and stir in the heavy cream, vanilla extract, and salt. Be cautious, as the mixture may bubble up at this stage. Stir until smooth.
5. Pour the caramel over the shortbread: Once the caramel is ready, pour it carefully over the cooled shortbread base. Use a spatula to spread it evenly across the surface. Allow the caramel to cool to room temperature before placing it in the fridge for at least 1 hour to firm up.

Step 3: Prepare the Chocolate Topping

1. Melt the chocolate: Chop the dark chocolate into small pieces and place it in a heatproof bowl. In a small saucepan, melt the butter over low heat. Once the butter is melted, pour it over the chopped chocolate and stir until the chocolate is completely smooth and glossy.
2. Pour the chocolate over the caramel: Once the caramel layer has set, pour the melted chocolate over the caramel. Use a spatula to spread the chocolate evenly across the surface. Allow the chocolate to set at room temperature for about 30 minutes, or speed up the process by refrigerating it for 15 minutes.

Step 4: Slice and Serve

1. Remove from the pan: Once the chocolate has hardened, lift the shortbread from the pan using the parchment paper overhang.
2. Slice the cookies: Place the shortbread on a cutting board and use a sharp knife to cut it into squares or rectangles, depending on your preference.