Preheat oven to 200C/400F/Gas 6.
Grease two ovenproof ramekins with butter and set aside.
In a bowl, whisk the egg yolk into the cooled, melted chocolate.
In a clean bowl, whisk the egg whites until stiff peaks form when the whisk is removed.
Add the lemon juice and caster sugar to the egg whites and whisk again.
Add a third of the egg white mixture into the chocolate mixture and stir well.
Carefully fold the remaining egg white mixture into the chocolate to retain the volume of the egg whites.
Pour the mixture into the ramekins and place into the oven to bake for 8-10 minutes, or until risen and set.
Serve immediately, dusted with icing sugar.