Preheat oven to 325. Grease a 9x13 pan.
In a bowl, combine cracker crumbs, sugar, and melted butter. Press mixture firmly into the bottom of the baking pan (the back of a spatula works well for this).
In the bowl of a stand mixer fitted with the paddle attachment, beat the cream cheese and the sugar until smooth. Incorporate the eggs, followed by the pumpkin, pumpkin pie spice, vanilla, and salt.
Pour the mixture into another bowl. From the mixture measure 1-1/4c and return it to the bowl of the stand mixer. Fit the stand mixer with the whisk attachment.
To a small saucepan over low heat, add the chocolate and the butter. Stir until melted. Pour the melted chocolate into the bowl of the stand mixer and whip it into the 1-1/4c pumpkin cheesecake mixture you have reserved. Beat on medium-high until smooth.
Carefully pour the chocolate cheesecake mixture over the crust and very carefully spread evenly with an off-set spatula (the crust will lift from the bottom of the pan if you're not careful, just take your time and don't worry if a few crumbs mix into the chocolate). Bake for 15 minutes.
Pour the remaining pumpkin cheesecake mixture over the baked chocolate layer. Bake for 35-45 minutes more or until set in the center. Cool on a wire rack for 30 minutes.
In a bowl, whisk together the sour cream and sugar until smooth. Spread over the cooled cheesecake using an off-set spatula. Cool bars completely, then chill for at least three hours in the refrigerator.
Cut into bars (for best presentation, wipe off the knife between each cut).