Melt chocolate over hot water. Using a thin small clear 4-sided cellophane bag cut the top off to desired height. Un-crease the bag so it stands up and open. Using a paintbrush, coat the inside of the bag well with chocolate. Put in the freezer to set up until very hard, about 30 minutes. Cut the bag a little and pull it away from the chocolate. Store chilled until ready to serve. To serve, fill it with white chocolate mousse then lay it on a plate on its side so it looks like it's spilling over. Garnish it with a cascade of mixed berries and a mint sprig. You can fill it with truffles, too.
Two days before you plan to serve the dessert, in a small saucepan, heat the cream over medium heat just until it boils. Immediately turn off the heat. Place the chocolate in a medium bowl. Pour the hot cream over the chocolate and whisk until melted and smooth. Strain into another bowl, cover, and refrigerate overnight.
The next day, remove the mixture from the refrigerator and, using a mixer fitted with a whisk attachment or a hand mixer, whip it into fluffy, soft peaks. Return to the refrigerator.
In a clean dry bowl, whip the egg whites until soft peaks form, then add the sugar and continue whipping until glossy and stiff, about 30 seconds more. Fold into the white chocolate mixture, and then spoon the mixture (or pipe through a pastry bag) into glasses or chocolate bags.