The Wedding cake frosting is the one I would use for your cupcake icing.
The cake decorating frosting (also known as royal icing)on my site is good for making snowflakes, flowers, lace, string work and for intricate piping onto fondant.
They both need refrigeration and to be covered well. The decorating frosting has egg white in it and the butter cream does too.It will stay fresh in the refrigerate for a few weeks. The decorator icing is best used right away but you can re whip it if needed. You can freeze the buttercream and it will keep longer.
