Preheat the oven to 350°F (175°C). Position a rack in the center. Lightly grease a 9 × 13-inch metal baking pan and line it with parchment paper, leaving overhang on the long sides for easy removal.

In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar on medium speed for 2–3 minutes. The mixture should look creamy, smooth, and slightly lighter in color. Scrape down the bowl as needed.

Add the eggs one at a time, beating until fully incorporated after each addition. Mix in the vanilla extract. The mixture should look smooth and cohesive rather than curdled.

In a separate bowl, whisk together the 3 cups (375 g) all-purpose flour, baking soda, and kosher salt. Add the dry mixture to the butter mixture in two additions, mixing on low speed just until no dry flour remains. The dough should be thick, soft, and easy to press into the pan.

Press approximately two-thirds of the cookie dough evenly into the bottom of the prepared 9 × 13-inch pan. Use lightly dampened fingers or an offset spatula to create a level surface. Bake for 10–12 minutes, until the surface looks set but remains pale. The crust should not be fully baked at this stage.

While the base bakes, place the soft caramels and heavy cream in a medium saucepan over low heat. Stir frequently for about 4–6 minutes, until the caramels are completely melted and the mixture is smooth. Remove from the heat. Whisk in the 1 tablespoon (8 g) flour, vanilla extract, and fine salt until thoroughly combined. The caramel should be thick, glossy, and pourable.

Remove the partially baked crust from the oven: Carefully pour the warm caramel filling evenly over the surface, leaving approximately ¼ inch around the edges. Spread gently with the back of a spoon if necessary. Avoid scraping the pan too aggressively, which can disturb the soft cookie base.

Take small pieces of the remaining cookie dough and flatten them slightly between your fingers. Scatter them evenly over the caramel. It is fine if some caramel remains visible between the pieces. The dough will spread slightly during baking and create a rustic, crumbled top.

Return the pan to the oven and bake for another 22–25 minutes. The top should be lightly golden, the edges should look set, and the caramel may bubble gently in the exposed spaces. Avoid baking until the entire top is deeply browned, which can produce dry cookie layers and overly firm caramel.

Remove the pan from the oven: If using the optional flaky sea salt, sprinkle it lightly over the surface while the bars are still warm. The salt should be used sparingly so it balances rather than overwhelms the caramel.

Place the baking pan on a wire rack and allow the bars to cool completely for about 2 hours. Do not slice while warm. The caramel filling needs time to thicken and set enough to hold cleanly between the cookie layers. For especially neat slices, refrigerate the fully cooled pan for 20–30 minutes before cutting.

Use the parchment paper overhang to lift the cooled slab from the pan. Place it on a cutting board and cut into 24 bars using a sharp knife. Wipe the knife clean between cuts if caramel begins to collect on the blade. Serve at room temperature for the softest caramel texture.