In a food processor, whirl cookies into crumbs; you should have 2 cups. Pour into a 9-inch cheesecake pan with removable rim; add butter and mix. Press mixture over bottom and 1/2 inch up sides of pan. Bake in a 350° oven until crust looks dry, about 10 minutes.
In a 1- to 2-quart pan, bring 1/4 cup cream to a boil. Remove from heat, add 2 ounces chocolate, and stir until smooth.
In a large bowl, with a mixer, beat cream cheese, sugar, and cocoa until smooth. Add eggs, one at a time, beating well after each addition. Add chocolate mixture and vanilla; beat until blended. Scrape into crust; spread level.
Bake cheesecake in a 325° regular or 300° convection oven until center jiggles only slightly when pan is gently shaken, about 1 hour. Cool for 1 hour.
In a 1- to 2-quart pan, bring remaining 1/4 cup cream to a boil. Remove from heat, add remaining 4 ounces chocolate, and stir until smooth. Pour over cake and spread level. Cover and chill at least 8 hours or up to 1 day.
Run a knife between cake and pan rim; remove rim. Cut cake into wedges.