1. Preheat the oven to 375 degrees F. 2. For the crust, combine the graham-cracker crumbs, sugar and butter; mix well. 3. Press the mixture firmly into an ungreased 9-inch pie plate. Press flat onto the bottom only. 4. Bake for 8 minutes or until the edges are slightly brown. Reduce the oven temperature to 350 degrees. 5. For the filling, combine the cream cheese, sour cream, cornstarch, and sugar in the bowl of a mixer. Mix until the sugar has dissolved. 6. Add the butter and vanilla and blend until smooth. Be careful not to overmix, to the filling will become too fluffy and will crack when cooling. 7. Pour the filling over the crust. 8. Bake for 30-35 minutes, or until a knife inserted 1 inch from the edge comes out clean. 9. Cool for 1 hour. 10. For the topping, mix the sour cream and powdered sugar. Spread the mixture over the top of the cooled cheesecake. Chill or freeze until ready to eat.