Combine the graham cracker crumbs, brown sugar, cinnamon, and salt in a medium mixing bowl. Mix well with your hands. Add the melted butter, stir well with a fork, then rub the mixture together with your fingers until evenly mixed. Press the mixture with your fingertips to form a level layer on the bottom of the prepared pan with a lip extending about 1/4 inch up the sides. Bake for 8 minutes on the center oven rack. Cool on a rack. Reduce the oven temperature to 325 degrees F.
1 package (8 ounces) cream cheese, softened
2/3 cup plus 1 tablespoon sugar, divided
1 large egg plus 1 egg yolk, at room temperature
1 teaspoon vanilla extract
1 tablespoon cornstarch
1 teaspoon cinnamon
1 teaspoon ground ginger
1 teaspoon nutmeg
3/4 cup mashed or canned pumpkin
1/3 cup heavy cream
Using an electric mixer, beat the cream cheese until smooth, gradually adding the 2/3 cup of sugar. Beat in the egg and yolk, until smooth, then blend in the vanilla. Mix the cornstarch, spices, and additional 1 tablespoon of sugar in a separate small bowl. Slowly beat the spice mixture into the cream cheese mixture. Beat in the pumpkin and
heavy cream until evenly blended. Pour the filling over the crust. Bake on the center oven rack for 40 to 45 minutes. When done, the filling will have risen and it will not be loose or liquid when you move the pan. Cool on a rack.
For the topping (optional):
1 cup sour cream
1/4 cup sugar
While the bars are still slightly warm, combine the sour cream and sugar in a small saucepan. Warm the mixture over moderate heat, stirring constantly, until it becomes thin enough to pour. Do not overheat or boil. Pour the topping over the center of the bars. Immediately tilt the pan in all directions, to cover evenly. Finish cooling, then cover the pan with foil and chill for at least 4 hours before slicing.