Preheat your oven to 325°F (165°C). Line a 9×13-inch baking pan with parchment paper, leaving overhang on the long sides to lift the bars out later. Lightly spray the parchment with nonstick spray.

In a large mixing bowl, beat together the softened butter, granulated sugar, and brown sugar until smooth and creamy. Mix in the egg and vanilla until combined. In a separate bowl, whisk together the flour, baking powder, and fine sea salt. Add the dry ingredients to the butter mixture and stir until a soft, crumbly dough forms that holds together when pressed.

Transfer about 2/3 of the dough to the prepared pan. Press it firmly and evenly into the bottom of the pan, all the way to the corners. Chill the remaining 1/3 of the dough in the refrigerator; it will be used for the crumble topping.

Bake the base layer for 18–20 minutes, or until just starting to turn lightly golden around the edges. Do not bake until deeply browned. While the crust bakes, prepare the caramel filling.

In a medium saucepan, combine the butter, brown sugar, heavy cream, and light corn syrup. Cook over medium heat, stirring frequently, until the mixture begins to bubble gently around the edges. Once bubbling, cook for 3–4 minutes, stirring constantly, until slightly thickened and glossy. Remove from heat and stir in the vanilla and 1/2 teaspoon flaky sea salt. Taste carefully and add a bit more salt if a stronger sweet-and-salty flavor is desired. Let the caramel cool for about 5 minutes to thicken slightly.

Pour the warm caramel evenly over the hot, partially baked crust. If using, sprinkle chopped nuts and/or mini chocolate chips evenly over the caramel layer.

Remove the reserved dough from the refrigerator and crumble it evenly over the caramel layer. Do not press flat; leave small clumps and crumbs with some caramel peeking through.

Return the pan to the oven and bake for 22–28 minutes, or until the top is light golden and the edges are bubbling slightly. The center may look a bit soft but should not be liquid; it will firm up as it cools. If the top browns too quickly, tent the pan loosely with foil for the last 5 minutes.

Transfer the pan to a wire rack. While the bars are still warm, sprinkle the top with additional flaky sea salt. Allow the bars to cool completely in the pan, 2–3 hours at room temperature. For extra-neat slices, chill the cooled bars in the refrigerator for 45–60 minutes before cutting.

Use the parchment overhang to lift the entire slab of bars out of the pan. Place on a cutting board and slice into 16 large or up to 24 smaller bars, wiping the knife clean between cuts. Serve at room temperature.