1. Preheat the oven to 350°F (175°C) and line an 8×8-inch baking pan with parchment paper.

2. In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

3. Add the flour and salt, mixing until a crumbly dough forms.

4. Reserve about 1 cup of the dough for the topping and press the remaining dough firmly into the bottom of the prepared baking pan.

5. Bake the shortbread crust for 15 minutes or until lightly golden around the edges.

6. While the crust bakes, combine sweetened condensed milk, brown sugar, heavy cream, vanilla extract, and a small amount of butter in a saucepan over medium heat.

7. Stir continuously until the caramel thickens and becomes smooth and glossy.

8. Pour the caramel evenly over the partially baked crust.

9. Sprinkle the reserved crumb mixture over the caramel layer.

10. Return the pan to the oven and bake for 20 to 25 minutes until the top becomes lightly golden and the caramel bubbles gently.

11. Remove from the oven and allow the bars to cool completely before slicing.

12. Drizzle melted chocolate and sprinkle flaky sea salt on top if desired before serving.