Prep the pan and oven: Heat the oven to 325°F (163°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang on the long sides for easy lifting. Lightly grease the exposed sides.
Make the crust: Crush graham crackers into fine crumbs.
Mix 2 cups crumbs with 1/2 cup melted butter, 1/4 cup granulated sugar, and a pinch of salt. It should feel like damp sand. Press firmly and evenly into the bottom of the pan.
Par-bake the crust: Bake the crust for 8–10 minutes until set and lightly golden.
Cool for 5–10 minutes while you make the filling.
Beat the cream cheese: In a large bowl, beat 24 oz (680 g) softened cream cheese on medium speed until completely smooth, about 2 minutes. Scrape the bowl well.
Add sugar and sour cream: Mix in 3/4 cup granulated sugar, 1/3 cup brown sugar, 1/2 cup sour cream, and 1 teaspoon vanilla. Beat until creamy and lump-free.
Avoid whipping too much air into the batter.
Blend in eggs: Add 3 large eggs, one at a time, mixing on low until just combined. Do not overmix. Set aside 1 cup of this plain cheesecake batter in a small bowl.
Make the pumpkin layer: To the main bowl, add 1 cup pumpkin purée, 2 tablespoons flour (or 1 tablespoon cornstarch), 1 to 1 1/2 teaspoons pumpkin pie spice, and a pinch of salt.
Mix on low until evenly blended and silky.
Assemble: Pour the pumpkin cheesecake batter over the crust and smooth the top. Dollop the reserved plain batter over the surface in spoonfuls. Swirl gently with a butter knife for a marbled look.
Don’t over-swirl.
Bake: Bake at 325°F for 35–45 minutes. The edges should be set and the center should have a slight wobble. If it looks wet or sloshy, give it a few more minutes.
Cool slowly: Turn off the oven, crack the door, and let the bars sit inside for 15 minutes.
Then remove and cool on a rack until room temperature.
Chill: Cover and refrigerate for at least 4 hours, preferably overnight. This step is key for clean slices and the best texture.
Slice and serve: Lift the slab out using the parchment overhang. Use a warm, dry knife for clean cuts, wiping between slices.
Add whipped cream, caramel, or chopped pecans if you like.