This frosting recipe hardens solid. It is commonly used for decorating crafts such as Sugar Easter Eggs or wedding cakes.
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Royal Icing (UNEDIBLE)

Ingredients-
2 egg whites
2 teaspoons water
1/8 teaspoon cream of tartar
3 cups confectioners' sugar

Using an electric mixer on high, beat the egg whites, water and cream of tartar until frothy. Gradually add the sugar and continue beating until the mixture is of spreading consistency. You may need to add more sugar. The icing should be firm enough to hold a soft peak. Cover with a damp towel to prevent from drying out. May be stored in an airtight container for later use.

Hint: The slightest amount of grease or oil will
breakdown this frosting and make it unuseable. Be
sure your bowls and utensils are grease-free. For
this reason, it is best not to use plastic bowls, wood
spoons or soft rubber spatulas when working with
Royal Icing.