Put the chocolate, orange rind and juice and cream in a small bowl over a pan of gently simmering water and heat gently until the chocolate has melted. Leave to cool, then beat in the egg yolk.
Meanwhile, whisk the egg white until it forms soft peaks, add the sugar and whisk until stiff.
Gently fold the egg white into the chocolate mixture, then spoon into glasses or ramekins. Chill for about 1 hour and decorate with orange rind before serving.