Place the chocolate pieces in a bowl in a double boiler and melt over low heat, stirring to melt smoothly.
Place the egg yolks, sugar and orange zest in a bowl in a double boiler and whisk intermittently over low heat until thickened and pale, approximately 7 minutes. Set aside to cool to room temperature.
Meanwhile, whip the heavy cream until soft peaks have just formed.
Add ½ the yoke mixture into the chocolate and fold to combine until smooth, the chocolate will become stiffer almost immediately, if the eggs and chocolate aren't even enough in temperature.
Add the remaining yoke mixture to the chocolate and continue to fold. Do not fear if the chocolate is stiff due to the combination of the yoke mixture, the mousse will still turn out perfect.
Spoon the chocolate into the whipped cream and fold through until well combined and most of the chocolate lumps have been smoothed out as you incorporate into the cream. The cream should resemble thickened almost curdled whipped chocolate cream.
Lastly, whip the egg whites until you reach soft peaks.
Add ⅓ of the chocolate cream and carefully fold through the egg whites so as not to lose volume.
Spoon the mousse into a serving dish and refrigerate for minimum 1 hour, but this is best served after chilling for 8 hours.
Serve the fluffy mousse into individual serving bowls.
Dig in!