In a medium saucepan, bring the cream to a boil. Remove from the heat. In a medium bowl, whisk half of the warm cream with the egg yolks. Return the cream to the saucepan and whisk constantly over moderately low heat, until an instant-read thermometer registers 160°. Remove from the heat and immediately stir in the chocolate, butter and vanilla and whisk until smooth. Let the filling cool to room temperature.