Preheat the oven to 350°F (175°C). Line a 9×9-inch square pan with parchment, leaving an overhang to lift the bars.

Make the shortbread: beat the softened butter with both sugars until pale and fluffy, about 2-3 minutes. Mix in vanilla. Add flour and salt and pulse or stir until the dough comes together.

Press the dough evenly into the prepared pan. Use the bottom of a measuring cup to smooth the surface.

Bake the crust for 15-18 minutes, until edges are just turning golden. Remove and let cool slightly while you make the caramel.

Place sugar in a medium, heavy-bottomed saucepan over medium heat. Let it melt without stirring; swirl the pan gently or use a heatproof spatula to encourage even melting.

When sugar is a deep amber, add the butter carefully (it will bubble). Stir until the butter melts, then slowly pour in the warmed cream while stirring.

Boil gently for 1-2 minutes until slightly thickened. Remove from heat and stir in salt. Let the caramel cool for 3-5 minutes so it’s warm but not scalding.

Pour the caramel over the shortbread in an even layer. Smooth gently and tap the pan to remove air pockets.

Chill the pan in the refrigerator for at least 2 hours until the caramel firms.

Melt the chocolate using a double boiler or microwave in 20-second bursts, stirring between, until smooth. Stir in oil if using.

Drizzle or spread the chocolate over the chilled caramel. Sprinkle a few sea salt flakes on top.

Let the chocolate set at room temperature or chill briefly until firm. Use the parchment overhang to lift the slab and cut into squares.