In a medium saucepan, heat the granulated sugar over medium heat. Stir the sugar occasionally with a rubber spatula until it forms clumps and subsequently melts into an amber-colored liquid.

Once the sugar has fully melted and there are no lumps, carefully add the butter to the saucepan. The mixture will vigorously boil; continue stirring until the butter is mostly integrated into the mixture.

Reduce the heat to low and incorporate the heavy cream into the mixture, whisking until the sauce becomes smooth and uniform.

Remove the saucepan from the heat, stir in the coarse salt, and set aside to cool completely.

Preheat the oven to 325°F (160°C). Prepare an 8x8 inch (20x20 cm) baking pan by lining it with parchment paper, ensuring an overhang on the edges for easy removal of the bars.

In a medium-sized bowl, combine the melted unsalted butter with the granulated sugar.

Gradually add the all-purpose flour, cornstarch, and salt to the butter and sugar mixture, ensuring that there are no lumps. The dough should be thick and crumbly.

Firmly press slightly more than half of this dough into the bottom of the prepared baking pan. Refrigerate the remaining dough.

Bake the base layer for approximately 20-25 minutes, or until the surface is set and begins to turn golden. Maintain the oven temperature after removing the base layer.

Allow the base layer to cool for 20-30 minutes, which is essential for the subsequent layers.

Spread ⅔ cup of the salted caramel sauce evenly over the cooled base. If using the prepared caramel sauce from this recipe, note that not all of it may be required.

Add the light brown sugar to the remaining dough, mixing until it achieves a crumbly texture. Evenly sprinkle this crumbly mixture over the caramel layer, allowing some caramel to remain visible.

Return the pan to the oven and bake for an additional 25-30 minutes, or until the crumble topping is golden and the caramel layer visibly bubbles.

Remove the baking pan from the oven and allow the bars to cool completely in the pan, which will take at least 3 hours. For quicker cooling, the pan may be placed in the refrigerator.

To serve, lift the bars out of the pan using the overhanging parchment paper, transfer to a cutting board, and slice into desired portions using a sharp knife.