Preheat oven to 350°F (175°C) and line a 9x13-inch baking pan with parchment paper, leaving overhang on sides.

Combine flour, sugar, and salt in a bowl. Add cold cubed butter and work it in until mixture resembles coarse crumbs.

Add vanilla extract and mix until dough comes together. Press firmly and evenly into prepared pan.

Bake shortbread base for 20-25 minutes until lightly golden. Remove and let cool while preparing caramel.

In a saucepan over medium heat, combine butter, brown sugar, and corn syrup. Stir constantly until butter melts and sugar dissolves.

Add sweetened condensed milk and continue stirring constantly for 8-10 minutes until mixture thickens and darkens to caramel color.

Remove from heat and stir in vanilla extract and sea salt. Pour hot caramel over baked shortbread base and spread evenly.

Let cool at room temperature for 15 minutes, then refrigerate for at least 2 hours until caramel is completely set.

Melt chocolate chips and butter together in microwave or double boiler, stirring until smooth.

Pour melted chocolate over set caramel layer and spread evenly. Sprinkle with flaky sea salt if desired.

Refrigerate for 1 hour until chocolate is set. Lift from pan using parchment overhang and cut into bars with sharp knife.

Let bars sit at room temperature 10-15 minutes before serving for best texture. Store in airtight container in refrigerator.