Melt the chocolate and butter in a double boiler or in a Pyrex glass bowl set over lightly simmering water. Do not let the bottom of the bowl touch the water. Stir until chocolate and butter have melted and remove from heat to cool, stirring occasionally while it rests.
In a medium-sized bowl, whip the egg whites with a pinch of salt on high until soft peaks form. Slowly add in the sugar while continuing to beat on high speed until stiff peaks form. This could take up to 4 minutes to achieve the right consistency. Once you get there, set it aside.
In a second bowl, beat whipping cream with icing sugar until stiff peaks form. Do not over whip.
By the time you have finished whipping the cream, the chocolate should be cooled enough to touch. Fold in whipped cream in two additions. Once combined, fold in the whipped egg whites until combined and no white is showing. The mixture will be quite soft.
Spoon into your serving dishes, cover with plastic wrap and chill for 3-4 hours or overnight to set.
When ready to serve top with extra whipped cream and your favourite garnishes.