Preheat oven to 350 degrees and prepare a water bath for your for your 8" x 8" baking pan (if using. See note below). Line baking pan with parchment or foil leaving an overhang on all sides. Set aside.
For the crust:
Stir together graham cracker crumbs and sugar. Pour melted butter into mixture and stir to combine. Dump into baking pan and evenly press crumb mixture into pan to form crust. Place in refrigerator to set while preparing cheesecake.
For the cheesecake:
Place cream cheese and sugar in a large bowl. Beat on medium high speed until smooth. Add vanilla and eggs and beat until combined. Stir in sour cream. Remove 1 3/4 cups batter from bowl and evenly spread over crust.
Add pumpkin puree and spices to the remaining batter and stir until thoroughly combined. Carefully spread pumpkin layer over cheesecake layer in baking pan.
Place baking pan in water bath in the oven and bake for 55-60 minutes. Turn off the oven, prop the oven door open with the handle of a wooden spoon and allow to cool for another hour.
Remove bars from water bath and refrigerate for at least 4 hours.
When ready to serve, lift bars from pan using overlapping parchment or foil. Cut cleanly using a hot knife (run under hot water and dried). Sprinkle bars with chopped chocolate, if desired.
The water bath is optional but I find it helps prevent cracking on top of the cheesecake bars. It's pretty easy to do. Pick a larger pan that will hold your baking pan inside it. Fill the larger pan with enough water so it comes halfway up the side of the baking pan when nestled inside it.
The bars can be made up to two days in advance. Simply remove the bars from the pan and cut into squares when ready to serve.