Line the tins with plastic film. Melt the chocolate over a water bath. Brush the inside of the tins with the melted chocolate. Refrigerate until set. Turn the tins upside down and remove the plastic film.
Put the After Eight, water and butter in a bowl and melt over a water bath. Stir in the egg yolk. Beat the egg white with the Siúcra/McKinney's Granulated Sugar until stiff peaks form, and fold into the chocolate mixture.
Fill the chocolate cases with the mousse. If desired, decorate with sugar sprinkles, for instance small sugar hearts or silver balls.
*Suggestion: use white After Eight if the chocolate cases are dark and dark After Eight if the chocolate cases are white.