Combine butter and chocolate wafer crumbs. Press into the bottom of a 10" springform pan.
Bake for 10 minutes at 325 degrees F.
Filling:
In a large mixer bowl, beat cream cheese and sugar until light and fluffy. Add eggs one at a time while beating. Stir in sour cream, Baileys, and vanilla.
Pour 2/3 of the cheesecake mixture over the crust. To the remaining mixture stir the melted chocolate. Spoon the chocolate mixture by large spoonfuls over the plain cheesecake mixture. Run a knife through it a few times to create a marbled effect.
Bake at 325 degrees F. for about 1 and 1/2 hours, until the center is raised and set. Let cool to room temperature.
Topping:
Coarsely chop the chocolate. Mix with whipping cream and butter and melt (on the stove or in microwave). Stir in vanilla. Spread over top of the cheesecake.