Preheat oven to 325ºF. Combine chocolate wafer cookie crumbs with butter and sugar. Firmly press the mixture into bottom and 1" up sides of a 9" springform pan. Bake for 10 minutes. Maintain oven temperature.
Microwave the chocolate on 50% power for 2 minutes in in a microwave-safe bowl. Stir, and microwave again on 50% power in 30-second intervals, stirring each time, until chocolate is 80% melted. At that point, remove from microwave and stir to finish melting. Cool to lukewarm temperature (it should still be pourable).
Combine cream cheese, sugar, cocoa powder and salt in stand mixer bowl and beat until just combined. Scrape sides and bottom of stand mixer, then beat in cooled chocolate and vanilla. Beat in eggs one at a time on low speed until just blended. Scrape mixture into the crust.
Bake cheesecake for about 55 minutes, until 150ºF or only the centre jiggles when you gently shake the pan. Run a knife around inside ring of pan, then allow cake to cool to room temperature before chilling it (still in the cake in pan) for 4-6 hours.
To make topping, whip cream in stand mixer until soft peaks form. Beat in sugar and vanilla.
To make chocolate drizzle, microwave chocolate and cream together in microwave-safe bowl at 50% power for 1 1/2 - 2 minutes, or until chocolate is 80% melted. Stir until smooth. Cool to room temperature before drizzling over whipped cream.