Preheat the oven to 325°F. Line an 8-inch square baking pan with parchment paper, leaving some parchment hanging over the sides for easy removal.
In a large mixing bowl, beat the softened butter, granulated sugar, and powdered sugar until light and creamy, about 2–3 minutes.
Add the vanilla extract and mix until combined. Gradually add the flour and fine sea salt, mixing just until a soft dough forms. Do not overmix.
Set aside about one-third of the dough for the crumb topping. Press the remaining dough evenly into the bottom of the prepared pan.
Bake the crust for about 15 minutes, or until the edges are just beginning to turn lightly golden.
While the crust bakes, place the unwrapped caramel candies and heavy cream in a saucepan. Heat over low to medium heat, stirring frequently, until completely melted and smooth.
Carefully pour the warm caramel over the partially baked crust. Spread it gently into an even layer using a rubber spatula.
Crumble the reserved shortbread dough evenly over the caramel layer. Keep the dough pieces loose rather than pressing them into the caramel.
Return the pan to the oven and bake for 25–30 minutes, or until the topping is lightly golden and the caramel is bubbling gently around the edges.
Remove the bars from the oven and immediately sprinkle the flaky sea salt over the top.
Allow the bars to cool completely in the pan. For cleaner slices, refrigerate them until the caramel has set, then lift them out using the parchment paper and cut into 16 squares.