Preheat oven to 350 degrees F (175 degrees C). Lightly grease an 8-inch square baking dish.
Mix flour and brown sugar in a bowl. Cut butter into flour mixture until small crumbs form. Stir pecans into the crumbs. Set 3/4 cup of the mixture aside for topping the bars; press remainder of the mixture into the bottom of the prepared baking dish.
Bake crust in preheated oven until lightly browned, about 15 minutes; remove from oven and set aside to cool slightly.
Beat cream cheese, white sugar, pumpkin puree, eggs, vanilla extract, allspice, and cinnamon together in a bowl until smooth; pour over the cooled crust. Sprinkle reserved topping over the cream cheese mixture.
Return dish to oven and continue baking until the topping browns, 30 to 35 minutes.