Preheat the oven to 325F (160C) degrees. Line an 8x8 inch (20x20 cm) baking pan with parchment paper so that there's an overhang around the edges.
In a medium bowl, whisk together the melted butter and granulated sugar. (You'll use the brown sugar later).
Stir in the flour, cornstarch and salt until there are no lumps. The dough will be thick and somewhat crumbly.
Press a little more than half of the dough into the bottom of the prepared pan. Place the remaining dough in the fridge.
Bake the base layer in the preheated oven for about 20-25 minutes, or until the top looks set and is beginning to golden. (Leave the oven turned on)
Remove from the oven and cool for 20-30 minutes - this is necessary!
After the base has cooled for 20 minutes, spread ⅔ cup salted caramel sauce on top. If you made your own caramel using the recipe above, you won't need all the caramel you made.
With the remaining dough, mix in the 2 tablespoons of brown sugar. It should be quite crumbly.
Crumble the remaining dough over the caramel layer. You will still see caramel through. I like to make larger crumbles, as they're less likely to dissolve into the caramel.
Return to the oven (still at 325F) and bake for 25-30 minutes or until the crumble on top is golden and no longer looks doughy. The caramel will bubble, which is expected.
Remove the pan from the oven. Cool the bars fully in the pan - they'll take at least 3 hours. You can place them in the fridge to speed up the process.
When ready to slice, lift the bars out of the pan using the parchment paper and place on a cutting board. Slice the bars using a sharp knife (not a table knife).