Make sure the flowers are clean and thoroughly dry before frosting. Put the egg white in a small bowl and beat lightly with a fork. Put the sugar in a separate bowl.
Using your fingers or a soft paintbrush, coat all the petals on both sides with the egg white. Dust plenty of sugar over the flowers until evenly coated. Transfer to a sheet of nonstick baking parchment and leave for at least 1 hour until firm.
Spread the chocolate frosting over the tops of the cooled cakes using a small palette knife. Decorate the top of each with the frosted flowers. Tie a length of ribbon around each paper cake case to decorate, finishing it with a ribbon bow, if using.