Combine flour and brown sugar in a medium bowl. Cut in the butter with a fork to make a crumb mixture. Stir in the nuts. Set aside 3/4 cup of this mixture for the topping.
Press the remaining mixture into the bottom of a 9x9 baking pan.
Bake in a a preheated 350 F oven for 13-15 minutes until light golden brown. Cool for about 15 minutes.
While cooling the crust, combine the cream cheese, granulated sugar, pumpkin, eggs, cinnamon, allspice & vanilla in a large mixing bowl. Blend with a mixer on medium until smooth. Pour over the cooled crust. Sprinkle with the reserved topping, and bake an additional 30-35 minutes at 350 F.
Cool before cutting into squares. Store in the refrigerator, but serve at room temperature. You could add a dollop of whipped cream also!