First Step: Prep your baking pan and crust. Preheat oven to 325°F (160°C) or 350°F (175°C) depending on your desired baking style. Line a 9×13 inch (or 9×9 inch for thicker bars) baking pan with foil or parchment paper, allowing overhang for easy removal. In a bowl, mix graham cracker crumbs with sugars and melted butter until evenly moistened. Firmly press the mixture into the bottom and slightly up the sides of the prepared pan. Bake for 10-15 minutes until crust is golden and fragrant, then set aside to cool slightly.

Second Step: Prepare pumpkin filling. In a medium bowl, combine pumpkin puree, pumpkin pie spice, cinnamon, and flour. Set this mixture aside.

Third Step: Make cream cheese batter. In a large bowl, beat softened cream cheese with sugars until smooth and creamy. Add vanilla extract, sour cream, and salt; mix thoroughly. Beat in eggs one at a time gently until just combined, avoiding overmixing to prevent cracking during baking.

Fourth Step: Combine and layer fillings. Reserve about 1½ cups of plain cream cheese batter. Fold the pumpkin mixture into the remaining cream cheese batter to create the spiced pumpkin filling. Alternately drop scoops of the plain and pumpkin batters over the crust, then use a knife gently to swirl the fillings, creating a marbled effect without disturbing the crust.

Fifth Step: Bake and cool. Bake for 35-45 minutes (350°F) or 40-45 minutes (325°F) until the edges are set and the center jiggles slightly but is not liquid. Do not open the oven door during baking to avoid cracks. Once baked, cool bars at room temperature on a wire rack, then refrigerate at least 6 to 8 hours or overnight to fully set before slicing.

Final Step: Slice and serve. Lift the bars out using foil overhang, slice into squares, and serve chilled. For dietary modifications, incorporate the appropriate ingredient substitutions suggested above to maintain flavor and texture.