Melt the chocolate in top of a clean double boiler over simmering water, stirring until smooth. Set aside to cool completely.
Using an electric mixer, beat the cream and sugar in a large bowl until soft peaks form. Set the whipped cream aside.
Using an electric mixer, beat the mascarpone cheese, melted chocolate and vanilla in a large bowl just until smooth and glossy, about 30 seconds (do not overmix) or fold in the mixture by hand.
Fold the whipped cream into the chocolate mixture in 2 batches.
Pipe immediately, or cover and refrigerate overnight. If chilled, allow to stand at room temperature 1 hour before piping. Transfer the chocolate mascarpone cream to a pastry bag fitted with a large star tip. Pipe.