Make the Shortbread Base:

1. Preheat and Prep: Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal later. Lightly grease the parchment.
2. Cream Butter and Sugar: In a large bowl, beat the softened butter and sugar together with a hand mixer or stand mixer on medium speed until light and fluffy, about 2-3 minutes.
3. Add Dry Ingredients: Add the flour and salt. Mix on low speed until the mixture comes together into a soft dough. It will look crumbly at first but will eventually form a cohesive dough.
4. Press and Bake: Press the dough evenly into the bottom of the prepared pan. Use your hands or the bottom of a measuring cup to create an even, compact layer. Prick the surface all over with a fork (this prevents bubbling).
5. Bake: Bake for 20-25 minutes, until the edges are lightly golden brown. The center may look slightly underdone, but it will continue to firm up. Remove from oven and let cool slightly while you prepare the caramel.

Make the Caramel Layer:

6. Combine Caramel Ingredients: In a medium saucepan, combine the butter, brown sugar, sweetened condensed milk, and corn syrup. Place over medium heat.
7. Cook the Caramel: Stir constantly with a wooden spoon or heat-resistant spatula. Bring the mixture to a gentle boil, then reduce heat to medium-low. Continue cooking, stirring constantly, for 5-7 minutes until the mixture thickens, darkens slightly in color, and reaches 225°F on a candy thermometer (soft ball stage). The caramel should coat the back of your spoon thickly.
8. Add Vanilla: Remove from heat and stir in the vanilla extract. Be careful—it may bubble up!
9. Pour and Spread: Immediately pour the hot caramel over the baked shortbread base. Use an offset spatula to spread it evenly into all corners. Let cool at room temperature for about 30 minutes, then refrigerate for at least 1 hour until firm.

Add the Chocolate Layer:

10. Melt Chocolate: Place the chocolate chips and coconut oil (or butter) in a microwave-safe bowl. Microwave in 30-second intervals, stirring between each, until completely melted and smooth (usually 1-2 minutes total). Alternatively, melt in a double boiler.
11. Spread Chocolate: Pour the melted chocolate over the cooled caramel layer. Use an offset spatula or the back of a spoon to spread it evenly to the edges.
12. Optional Salt: If desired, sprinkle flaky sea salt over the chocolate while it’s still wet.
13. Chill: Refrigerate for at least 2 hours, or until the chocolate is completely set and firm.

Cut and Serve:

14. Remove and Cut: Using the parchment overhang, lift the entire block out of the pan and place on a cutting board. Use a sharp knife to cut into squares or bars. For the cleanest cuts, run your knife under hot water and wipe dry between each cut, or let the bars sit at room temperature for 10 minutes before cutting.