Preheat oven to 350F and line a 9x13 pan with parchment. Make sure the parchment covers the bottom and comes up the sides so the bars lift out cleanly.

Combine all-purpose flour, powdered sugar, cornstarch, cinnamon, and salt, then cut in cold unsalted butter until the dough comes together. Stop mixing as soon as you can press the dough into cohesive clumps with no dry streaks.

Reserve 1 cup of the shortbread dough for the crumble topping and press the remaining dough firmly into the pan. Press evenly to form a compact, flat layer.

Bake the crust for 15 minutes until pale golden. Keep an eye on the edges so they set without browning too dark.

Whisk pumpkin puree, caramel sauce, heavy cream, brown sugar, pumpkin pie spice, vanilla, and eggs until smooth. Mix until no egg streaks remain and the filling looks glossy.

Pour the filling over the hot crust. Spread gently to reach the corners.

Crumble the reserved dough over the top. Use your fingers to make small, uneven pieces for a rustic texture.

Bake 22-25 more minutes at 350F until the filling is set and the crumble looks lightly golden. The center should look firm when you gently nudge the pan.

Sprinkle flaky sea salt over the bars immediately after baking. Watch it melt slightly on warm spots, then cling as it cools.

Cool completely before cutting. Lift out using the parchment and slice when fully cool so the filling holds cleanly.