Make crust: Invert bottom of a 9-inch springform pan (to create a flat bottom, which will make it easier to remove cake from pan), then lock on side and butter pan.
Stir together crumbs, pecans, sugars, and butter in a bowl until combined well. Press crumb mixture evenly onto bottom and ½ inch up side of pan, then chill crust 1 hour.
Make filling and bake cheesecake: Put oven rack in middle position and preheat oven to 350F. Whisk together pumpkin, eggs, brown sugar, cream, vanilla, and liqueur (if using) in a bowl until combined.
Stir together granulated sugar, cornstarch, cinnamon, nutmeg, ginger and salt in a large bowl. Add cream cheese and beat with an electric mixer at high speed until creamy and smooth, about 3 minutes. Reduce speed to medium, then add pumpkin mixture and beat until smooth.
Pour filling into crust, smoothing top, then put springform pan in a shallow baking pan (in case springform leaks). Bake until center is just set, 50-60 minutes. Transfer to a rack and cool 5 minutes. (Leave oven on).
Make topping: Whisk together sour cream, sugar, and liqueur (if using) in a bowl, then spread on top of cheesecake and bake 5 minutes. Cool cheesecake completely in pan on rack, about 3 hours. Chill, covered, until cold, at least 4 hours. Remove side of pan and bring to room temp before serving.