Decorate the glasses for an extra special touch. Put the egg white and the sugar in two separate small bowls. Dip the rims of the glasses in the egg white and then in the sugar, so you have an even coating on the rim of each glass. Leave to set.
Mix the grape juice, cointreau and lime cordial in a jug. Chill until needed.
Divide the cocktail base between your glasses to come about ⅓ of the way up the glass. Top up with chilled sparkling wine or Champagne and serve.