Preheat oven to 325°F (165°C) with rack in the middle position. Line a 9×13 inch (23×33cm) light-colored aluminum baking pan with parchment paper leaving a 2-inch overhang on both long sides.

Whisk together 240g all-purpose flour, 100g granulated white sugar, and ½ teaspoon fine sea salt in a large bowl until combined. Add 170g cold cubed unsalted butter and cut in with a pastry cutter or two forks until the mixture resembles coarse damp sand with some pea-sized butter pieces remaining.

Tip the shortbread mixture into the prepared pan and press firmly and evenly into the bottom using fingertips and the flat bottom of a dry measuring cup until approximately ½ inch (1.25cm) thick. Dock the surface with a fork 15–20 times across the surface.

Bake on the middle rack for 18–22 minutes until the surface is completely matte and dry and the edges are just barely beginning to show a faint pale gold. Remove from the oven and cool on a wire rack for exactly 15 minutes before adding the caramel.

While the base cools, clip a candy thermometer to the side of a medium saucepan with the bulb not touching the bottom. Add 200g packed dark brown sugar, 113g unsalted butter cut into tablespoons, and 80ml heavy whipping cream. Cook over medium heat stirring constantly until the butter melts and sugar dissolves and the mixture comes to a full boil — approximately 3–4 minutes.

Once at a full boil stop stirring. Allow to boil undisturbed until the thermometer reads 235–240°F (113–115°C) — approximately 4–6 minutes. Remove from heat immediately. Stir in 1 teaspoon pure vanilla extract and ¼ teaspoon fine sea salt.

Pour the hot caramel over the cooled shortbread base. Tilt the pan gently in all four directions to spread the caramel to the edges. Transfer to the refrigerator and chill for 30 minutes until the caramel is fully set and firm to a light touch at the center.

Place 255g bittersweet chocolate in a medium heat-safe bowl. Heat 120ml heavy whipping cream in a small saucepan over medium heat until just simmering — small bubbles at the edges, not a full boil. Pour over the chocolate and let sit undisturbed for 2 minutes. Whisk from the center outward until completely smooth and glossy. Cool for 3–4 minutes until slightly thickened but still pourable.

Transfer to the refrigerator and chill for a minimum of 2 hours until the ganache is completely firm throughout. Remove from the refrigerator and scatter 1½ teaspoons Maldon flaked sea salt evenly across the surface pressing each flake gently to adhere. Rest at room temperature for 10 minutes before cutting.

Use the parchment overhang to lift the slab onto a cutting board. Using a sharp chef's knife wiped clean between every cut, cut into 24 bars — 4 rows by 6 columns. Serve at room temperature.