Melt the butter, sugar & orange liqueur in a small saucepan over low heat, mixing until sugar has dissolved.
Add chocolate, mix to combine & melt the chocolate.
Add the orange rind & set aside to cool while you continue (see notes).
Whip the cream to soft peaks & set aside.
Whip the egg whites to soft peaks.
Add egg yolks to chocolate & mix to combine.
Fold together the whipped egg whites & chocolate mixture, then fold in whipped cream.
Pour mixture into a shallow container and refrigerate for 2 - 4 hrs or until set (see notes).
Serve immediately.
Notes
At the point of adding the chocolate & orange rind to the melted butter mixture, if the mixture starts to look a little grainy, I just put the saucepan over a light heat for a few seconds and mix until smooth. Depending on how finely grated your rind is, this will still show slightly in the mixture.2 hours chilling time is a minimum in my opinion for serving, but I much rather the mousse after 4 hours in the fridge - this makes the mousse quite sticky and fudgy.If you are not a fan of orange you can just leave out the liqueur & rind, the mousse still tastes divine. You could also substitute a different liqueur if you wish.*Please note that the amount of calories per serve is provided as a guide only, as ingredients and cooking methods can vary greatly*