Preheat the oven to 350°F.
Line a 9x13 baking pan with parchment paper, leaving an overhang on each end to make removing the baked bars easier. Set aside.
In a mixing bowl, combine the graham cracker crumbs and butter. Mix to make a thick dough, and then add the milk. The mixture will become softer and stickier. Spread it out in the prepared baking pan, pressing it evenly into the bottom of the pan. Set aside.
In a separate bowl, combine the cream cheese, sugar, and vanilla. Beat with a hand mixer until smooth and creamy. Add in the eggs, one at a time, beating until well combined.
Whisk the pumpkin puree and spices into the cream cheese mixture. Pour over the crust, and bake for 15 minutes.
In the meantime, prepare the streusel. In a clean mixing bowl, combine the brown sugar, all-purpose flour, rolled oats, chilled butter, vanilla, and cinnamon. Mix by hand or with a wooden spoon to make the streusel mixture.
When the 15 minutes are up, take the pumpkin bars out of the oven and crumble the streusel all over the top of the bars. Return the bars to the oven, and bake for another 35 - 40 minutes, until the streusel is golden-brown.
Take the bars out of the oven, and let them cool to room temperature. Then place the pan in the refrigerator to chill for several hours.
Once chilled, cut the pumpkin bars into large squares, using a sharp knife.
Optional: Drizzle the bars with cream cheese frosting or caramel topping before serving.