Position oven rack in middle of oven; preheat oven to 350°.
Butter a 9-inch springform pan and wrap the outside tightly with heavy-duty foil.
Have a roasting pan ready, and put a kettle of water on to boil for the water bath.
Make the crust: pulse the chocolate wafers in a food processor until finely ground.
With the motor running, slowly add the butter, and process just until blended.
Press mixture onto the bottom of the pan.
Bake crust for 8-10 minutes, until it is set; let cool on a wire rack.
Make the filling: melt the chocolate with the butter in a heatproof bowl set over a saucepan of about 1 1/2 inches of nearly simmering water, whisking until smooth.
Whisk in the cocoa powder.
Remove the bowl from the heat and let the mixture cool to room temperature.
Beat the cream cheese and sugar with an electric mixer, beginning on low speed and increasing to med-high speed, in a medium deep bowl until light and fluffy.
Add in the eggs one at a time, beating well after each addition.
Beat in the salt.
Beat in the chocolate mixture just until smooth, scraping down the sides of the bowl with a rubber spatula as needed.
Add in the sour cream and vanilla; beat until smooth, scraping sides of bowl as needed.
Transfer the filling to the springform pan.
Set the pan in the roasting pan, place it in the oven, and carefully pour in enough boiling water to reach halfway up the sides of the springform pan.
Bake for 45 minutes, or until the center is almost set but still slightly jiggly; do not overbake--the cheesecake will firm as it cools.
Remove the roasting pan from the oven and let the cheesecake cool in the water bath for 15 minutes.
Remove the springform pan from the water bath and let cool completely on a wire rack, then remove the foil and refrigerate the cheesecake, loosely covered, until thoroughly chilled, at least 12 hours, or overnight.
To serve--let the cheesecake stand at room temperature for 20 minutes.
Remove the pan sides, smooth the sides of the cheesecake with a table knife, and cut into wedges.