Adjust oven rack to middle position and heat oven to 350 degrees. Make foil sling for 13 by 9-inch baking pan by folding 2 long sheets of heavy-duty aluminum foil; first sheet should be 13 inches wide and second sheet should be 9 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan. Spray foil with vegetable oil spray.

Stir 3¾ cups (18¾ ounces) all-purpose flour, 2 cups packed (14 ounces) dark brown sugar, 24 tablespoons melted unsalted butter, 1½ teaspoons table salt, ½ teaspoons baking powder, and ¾ teaspoon baking soda in bowl until no dry spots remain and mixture forms clumps. Reserve one-third of flour mixture, about 2 cups, and refrigerate until ready to use (mixture will firm up as it chills).

Crumble remaining two-thirds of flour mixture evenly over bottom of prepared pan. Using your hands, press dough into even thickness. Bake until pale golden brown, 15 to 20 minutes. Transfer pan to wire rack. Let crust cool for about 20 minutes.

Combine 3 pounds Granny Smith apples, peeled and sliced ¼ inch thick; ⅓ cup packed (2⅓ ounces) dark brown sugar; 1 tablespoon grated lemon zest plus 2 tablespoons juice; 1 teaspoon ground cinnamon; ¼ teaspoon salt; and ½ cup jarred caramel sauce in large bowl, then spread into even layer over crust. Crumble reserved refrigerated flour mixture evenly over top. Bake until top is golden brown and crisp, 55 minutes to 1 hour 5 minutes.

Let bars cool in pan on wire rack for 1½ hours. Using sling, remove bars from pan and let cool another 2½ hours. Once cool, transfer bars to cutting board. Cut into 24 bars. Drizzle with remaining ¼ cup caramel sauce and sprinkle with 2 teaspoons flake sea salt. Serve. (Bars can be refrigerated, covered, for up to 2 days.)