FOR CRUST: Combine in bowl. If mixture seems too runny, chill slightly. Press into 9" springform pan. Chill until firm.
FOR MOUSE CAKE: Combine butter and sugar and beat until light and fluffy. Add eggs, 2 at a time, scraping sides each time. Beat 2 minutes. Add chocolate and beat until incorporated. Stir in cream and vanilla. Pour into crust. Refrigerate until firm, 6 hours or overnight.