Step 1: Preheat your oven to 350°F (175°C) and line an 8×8 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a large bowl, cream together 1 cup of softened butter and 1/2 cup granulated sugar until smooth. Gradually add the flour and 1/4 tsp salt, mixing until a crumbly dough forms.
Step 3: Press the dough firmly into the bottom of the prepared pan. Bake for 15-20 minutes or until the edges are lightly golden. Remove from the oven and set aside to cool slightly.
Step 4: In a medium saucepan, combine the 1/2 cup butter, brown sugar, condensed milk, and corn syrup. Stir over medium heat until the butter is melted and the sugar has dissolved.
Step 5: Bring the mixture to a gentle boil, stirring constantly. Continue to cook for 5-7 minutes or until the mixture thickens and turns a dark amber color. Stir in the vanilla extract.
Step 6: Pour the hot caramel over the baked shortbread base and spread it evenly. Sprinkle the flaky sea salt over the top while the caramel is still wet.
Step 7: Allow the bars to cool at room temperature for at least an hour, then refrigerate for 2 hours until the caramel is firm. Lift from the pan using the parchment paper and slice into squares.