Melt the chocolate and butter with the orange zest in a ban marie or mixing bowl over a pot of hot (not boiling) water. Once completely broken down (no grainy bits) pass the mixture through a sieve to remove the zest.
Add the egg yolks to the warm (not hot) mixture. Leave the mix in the fridge for ten minutes prior to this stage if necessary
Whisk the room temperature egg whites with a pinch of salt till stiff then gradually add the caster sugar to form thick glossy peaks.
Whip cream and vanilla essence to soft peaks.
Add a quarter of the whipped egg white mixture to the chocolate and fold in. Then add the chocolate mixture to the rest of the egg white mix folding gently this helps to keep the dessert light and fluffy.
Add the whipped cream and vanilla to the chocolate and egg white mixture folding gently all the time.
The mix can then be piped in moulds or directly into wine glasses for serving.
Ideally serve with whipped cream and chocolate shavings.