Cost per Serving:
Position a rack in middle of oven and preheat to 350°F. Butter a 9-inch springform pan and wrap outside tightly with aluminum foil.
Make crust: Pulse chocolate wafers in food processor until ground. Add butter and process just until blended. Press mixture onto bottom of pan and bake until set, 8 to 10 minutes. Cool on a wire rack.
Make filling: Melt chocolate with butter in a heatproof bowl set over a pan of nearly simmering water. Whisk in cocoa powder. Remove from heat and cool to room temperature. In a separate bowl, beat cream cheese and sugar until fluffy. Add eggs one at a time, beating well after each. Add sour cream, vanilla and salt, and beat until smooth, scraping down sides of bowl. Set aside 1 cup mixture. Whisk chocolate mixture into remaining cream cheese mixture.
Pour chocolate mixture into springform pan and top with reserved mixture. Swirl a knife through to marbleize. Set pan in a roasting pan, place in oven and carefully pour in enough boiling water to reach halfway up sides of springform pan. Bake for 55 minutes, or until center is almost set but jiggles slightly. Remove from oven and let cheesecake cool on a wire rack in water bath for 15 minutes, then remove pan to wire rack to cool.
Remove aluminum foil and refrigerate cheesecake, loosely covered, until chilled, at least 12 hours. To serve, let cheesecake stand at room temperature for 20 minutes. Remove pan sides, cut into wedges and serve.