1 1/2 lbs. (Three 8 oz. boxes) cream cheese, at room temperature
to quickly warm, remove the blocks from their packaging and microwave until soft, about 30 seconds)
1 16 oz. carton sour cream
1 tsp. vanilla extract
optional garnish: toasted or candied pecans
Adjust oven rack to middle position and heat oven to 375 degrees.
Heat butter and sugar in a saucepan until butter melts. Meanwhile, line the bottom of a 9"x13" baking pan with a 12"x20" sheet of foil so it hangs over the long sides of the pan (you'll use this as a "handle" to pull the cooked dessert from the pan). Arrange graham crackers over foil-lined pan bottom, cutting the final few with a knife so they'll fit snugly. Pour butter mixture over crackers; spread with knife to cover. Bake until butter-sugar mixture starts to harden, about 7 minutes. Remove from oven. Reduce temperature to 300 degrees.
As crust bakes, heat pumpkin, 1 1/4 cups brown sugar and spices in a medium pan over medium-high heat until mixture is sputtery hot. Process eggs on high speed in a blender. Transfer pumpkin to a pourable container. With blender running, slowly add pumpkin mixture; puree until smooth. Add cream cheese, one block at a time; puree until smooth. Pour mixture over crust; bake until set, about 35 minutes.
Meanwhile, mix sour cream, 1/2 cup dark brown sugar and vanilla. Remove cheesecake from oven. Pour sour cream mixture evenly over top; carefully spread so top is completely covered. Return to oven. Bake to set topping about 5 minutes longer (I FELT IT COULD'VE USED A FEW MORE MINUTES, AT YOUR DISCRETION). Cool to room temperature, then refrigerate (can be made up to 2 days ahead).
To serve, run a knife around the pan perimeter to loosen cake. Use foil handles to pull cake from pan. Cut into squares (up to 15) and garnish.