Step 1: Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a medium bowl, combine the all-purpose flour, granulated sugar, and salt. Whisk briefly to combine.
Step 3: Add the cold, cubed unsalted butter to the dry ingredients. Using a pastry blender, your fingertips, or a food processor, cut the butter into the flour mixture until it resembles coarse crumbs.
Step 4: Press the crumb mixture evenly into the bottom of the prepared baking pan. Use the bottom of a flat glass or your hands to get a compact, even layer.
Step 5: Bake the crust for 15-18 minutes, or until lightly golden brown around the edges. Set aside to cool slightly while you prepare the filling.
Step 6: In a large mixing bowl, whisk together the canned pumpkin puree, sweetened condensed milk, large eggs, pumpkin pie spice, and vanilla extract until smooth and well combined.
Step 7: Pour the pumpkin mixture evenly over the warm, pre-baked shortbread crust.
Step 8: Drizzle about half of the caramel sauce in lines or dollops over the pumpkin filling. Take a knife or skewer and gently swirl the caramel into the pumpkin mixture to create a marbled effect.
Step 9: Bake for 45-55 minutes, or until the center is just set and the edges are slightly puffed. A knife inserted near the center should come out mostly clean.
Step 10: Once baked, remove the pan from the oven and let it cool completely on a wire rack.
Step 11: Once cooled, place the pan in the refrigerator for at least 2-4 hours, or preferably overnight, to allow the pumpkin slice to firm up completely. Chilling makes it much easier to cut into clean squares.
Step 12: Before serving, use the parchment paper overhangs to lift the entire slab out of the pan. Drizzle the remaining caramel sauce generously over the top of the chilled pumpkin slice.
Step 13: Cut into 12-16 squares. A warm, sharp knife will give you the cleanest cuts. Clean the knife between each cut for perfect edges. Enjoy!