Line a 9×13-inch metal baking pan with parchment paper, leaving overhang on the long sides for easy removal. Lightly butter or spray the parchment and sides of the pan. Preheat the oven to 325°F (165°C).

In a large mixing bowl or stand mixer, cream the softened butter and granulated sugar together until pale and fluffy, about 2–3 minutes. Add the flour and salt, and mix on low just until the mixture looks like slightly clumpy sand and holds together when pressed between your fingers.

Tip the shortbread mixture into the prepared pan. Press it down firmly and evenly with clean hands or the bottom of a measuring cup, making sure to reach into the corners. Prick the surface a few times with a fork to prevent puffing. Bake for 22–25 minutes, or until the edges are just turning light golden and the top looks set. Cool slightly on a rack while you start the caramel; the base can be warm but not piping hot when you pour the caramel over.

In a medium heavy-bottomed saucepan, combine the butter, light brown sugar, sweetened condensed milk, and light corn syrup. Cook over medium heat, stirring constantly, until the butter melts and the mixture is smooth and fully combined.

Once the caramel mixture begins to bubble, reduce the heat to medium-low. Continue cooking, stirring constantly and scraping the bottom and corners of the pan, for 8–12 minutes, until it thickens and turns a deeper golden color. It should leave a trail when you drag a spatula through it and the bubbles will be slower and more lava-like. If using a candy thermometer, aim for 225–230°F (107–110°C) so the caramel sets thick but sliceable, not chewy.

Remove the saucepan from the heat and stir in the vanilla extract and fine sea salt until evenly combined.

Immediately pour the hot caramel over the warm shortbread base. Tilt the pan gently and use a spatula to spread it into an even layer. Let the pan sit at room temperature for 30–40 minutes, until the caramel has firmed slightly and is no longer very hot. You can chill it in the fridge for 20–30 minutes to speed this up, but do not freeze at this stage.

Place the chocolate and butter (or heavy cream) in a heatproof bowl. Melt in the microwave in 20–30 second bursts, stirring well between each, or use a double boiler on the stove. Stir until completely smooth and glossy; residual heat will melt any remaining small pieces.

When the caramel is set but still just slightly warm, pour the melted chocolate over the top. Gently tilt the pan and use an offset spatula to spread it into an even layer. If desired, sprinkle lightly with flaky sea salt while the chocolate is still wet.

Chill the pan in the refrigerator for 1–1.5 hours, until the chocolate is set but not rock hard. Remove from the fridge and let sit at room temperature for 15–20 minutes so the chocolate softens slightly. Use the parchment overhang to lift the whole slab onto a cutting board. Warm a sharp knife under hot water, dry it, then press straight down to cut into bars or squares, wiping the blade between cuts for clean layers.