Combine butter and chocolate wafer crumbs. Press into the bottom of a 10" springform pan.
Bake for 10 minutes at 325 degrees F.
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.
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.