Place the butter and chocolate (broken into pieces) into a microwave-safe bowl. Melt on 50% power in 30 second bursts until melted. Allow to cool.
Separate the eggs.
Beat the egg whites and caster sugar until firm peaks have formed. Set aside.
Whip the cream until stiff peaks form. Gently fold the egg yolks through with a rubber spatula - it's fine for the mixture to be streaky (don't over-mix).
Pour the runny (but cooled) chocolate into the cream/egg mixture. Very gently fold the chocolate through with the same rubber spatula. Don’t over-mix – again streaky mixture is fine!
Fold ¼ of the egg white mixture through the chocolate mixture.
Add the chocolate mixture to the bowl of egg whites and fold through until just combined (don’t over-mix).
Spoon into serving glasses or bowls and refrigerate until set (at least 4 hours).
Decorate with your choice of optional toppings - including chopped strawberries or raspberries, chocolate shavings, whipped cream, crushed peppermint crisp bar, crushed chocolate honeycomb.