Heat the oven to 350F and line an 8-inch square pan with parchment, leaving an overhang on two sides. Stir the graham crumbs, both sugars, and melted butter together until it looks like wet sand.
Press the mixture firmly and evenly into the bottom of the pan. Bake for 8 to 10 minutes, then set aside to cool slightly.
Make the Filling
Beat the softened cream cheese with both sugars until smooth and creamy. Mix in the sour cream and vanilla.
Add the eggs one at a time, beating just until combined. Stir in the pumpkin, pumpkin pie spice, and cinnamon until the color is even.
Bake and Cool
Pour the filling over the crust and smooth the top. Bake for 35 to 45 minutes, until the edges are set and the center still has a slight jiggle.
Cool at room temperature, then chill at least 4 hours before lifting out and cutting into squares.
Notes
Room temperature: Soften the cream cheese and bring the eggs to room temperature so the filling blends smooth and lump-free.
Don't overbake: Pull the bars when the center still jiggles slightly. They firm up as they chill.
Clean cuts: Chill fully, then cut with a sharp knife, wiping the blade between cuts.
Storage: Fridge 4 to 5 days, or freeze wrapped up to 2 months. Thaw overnight in the fridge.