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 to easily lift the bars later.
Step 2: In a medium bowl, combine the graham cracker crumbs and 1/4 cup granulated sugar. Pour in the melted butter and mix until the crumbs are evenly moistened.
Step 3: Press the crumb mixture firmly and evenly into the bottom of the prepared baking pan. Bake the crust for 8-10 minutes, or until lightly golden. Remove from oven and let cool slightly while you prepare the filling.
Step 4: In a large bowl, whisk together the pumpkin puree, evaporated milk, eggs, brown sugar, 1/4 cup granulated sugar, pumpkin pie spice, salt, and vanilla extract until smooth and well combined.
Step 5: Pour the pumpkin filling over the partially baked graham cracker crust in the pan.
Step 6: Return the pan to the oven and bake for 35-40 minutes, or until the edges are set and the center still has a slight wobble when gently shaken. A knife inserted near the center should come out clean.
Step 7: Remove the bars from the oven and let them cool completely on a wire rack at room temperature. This can take 1-2 hours.
Step 8: Once cooled, transfer the pan to the refrigerator and chill for at least 4 hours, or preferably overnight, before slicing. This is crucial for the bars to set properly.
Step 9: Once thoroughly chilled, use the parchment paper overhang to lift the entire slab of bars out of the pan. Place on a cutting board and slice into squares or rectangles. Serve chilled and enjoy your decadent fall treat!