Warm the chocolate in a bowl set over a pot of lightly simmering water. Remove from heat when only partially melted; stir to melt completely.
Beat the cream cheese, sugar, and vanilla until smooth. Beat in the chocolate, then the sour cream. Whip the heavy cream until firm; gently fold into chocolate filling.
Put the banana slices on the crust, top with the filling, and chill at least 2 hours. If you'd like, sprinkle with grated dark chocolate before serving.
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