In a medium-size saucepan, mix chocolate, cream and butter together until smooth. Remove from heat and let cool.
In a large bowl, beat egg whites until stiff peaks form. In another bowl, beat whipping cream and powder sugar until stiff peak form; fold into egg whites. Fold egg white/whipping cream mixture into cooled chocolate mixture.
Place the jocund base on a loosed bottom cake ring. Arrange the mango slices over the cake. Pour the chocolate mixture over and smooth top. Chill.
Make topping glaze:
Heat cream in a heavy saucepan until simmering point. Pour over chopped chocolate and stir until smooth. Set aside to cool completely, stirring occasionally. Pour over the top of the chilled cake, let it set and decorate with chocolate curls cocoa powder.