Whisk the ice cream and cocoa powder in a saucepan, making sure there are no lumps. Heat over medium heat until simmering. Put the chopped chocolate in a medium bowl and pour the heated ice cream over the top. Stir to evenly melt the chocolate, then let cool to warm.
Beat the egg whites with a pinch of salt until they hold stiff peaks. Whisk one-third of the whites into the chocolate base to lighten. Gently fold in the remaining egg whites.
Beat the heavy cream until it just holds medium-stiff peaks, then fold into the chocolate mixture. Divide the mousse among 4 serving bowls and chill until set, about 1 hour 30 minutes. Serve topped with the shaved chocolate.
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