Make crumb crust as directed in separate recipe, using chocolate wafer cookies instead of graham crackers.

Preheat oven to 350°F.

Cook sugar in a dry heavy saucepan over moderately low heat, stirring slowly with a fork, until melted and pale golden. Cook caramel without stirring, swirling pan, until deep golden. Remove from heat and carefully add heavy cream (mixture will vigorously steam and caramel will harden). Cook over moderately low heat, stirring, until caramel is dissolved.

Remove from heat and whisk in chocolate until smooth. Stir in sour cream.

Beat cream cheese with an electric mixer until fluffy, then beat in chocolate mixture on low speed.

Beat in eggs, 1 at a time, then vanilla, beating on low speed until each ingredient is incorporated and scraping down bowl between additions.

Put springform pan with crust in a shallow baking pan. Pour filling into crust and bake in baking pan (to catch drips) in middle of oven 55 minutes, or until cake is set 3 inches from edge but center is still slightly wobbly when pan is gently shaken.

Run a knife around top edge of cake to loosen and cool completely in springform pan on a rack. (Cake will continue to set as it cools.) Chill cake, loosely covered, at least 6 hours. Remove side of pan and transfer cake to a plate. Bring to room temperature before serving.

Cheesecake keeps, covered and chilled, 1 week.