Here’s the recipe from Martha Stewart in 2000 for Royal Icing. She used white icing to make her Christmas trees from stacks of sugar cookies. If you’re decorating cookies for children, pregnant women, or anyone with compromised immune systems, use meringue powder, found in the baking aisle of most markets, instead of raw egg whites.
1 box confectioners’ sugar (1 pound)
5 tablespoons meringue powder, or 2 large egg whites
1. In the bowl of an electric mixer fitted with the paddle attachment, combine sugar and meringue powder. Mixing on low speed, add a scant 1/2 cup water. For a thinner consistency, usually used for flooding, add more water. A thicker consistency is generally used for outlining and adding details. Mix until icing holds a ribbon-like trail on the surface of the mixture for 5 seconds when you raise the paddle.
Makes 2 and 1/3 cups icing.