Egg whites act as a stabilizer in this icing, allowing it to harden for decorating the cookies. Because the whites are not cooked, we prefer powdered egg whites, such as Just Whites, which are available in the baking section of most supermarkets.

1 (1-lb) box xxx sugar               4 t powdered egg whites (not reconstituted)
¹/₃ cup water                            1 T fresh lemon juice
1 t vanilla                                Food coloring (optional)

Beat together all ingredients except food coloring in a large bowl with an electric mixer at moderate speed until just combined, about 1 minute. Increase speed to high and beat icing, scraping down side of bowl occasionally, until it holds stiff peaks, about 3 minutes in a standing mixer or 4 to 5 minutes with a handheld. Beat in food coloring (if using). If you plan to spread (rather than pipe) icing on cookies, stir in more water, 1 teaspoon at a time, to thin to desired consistency. Put icing in pastry bag (if using) and pipe or spread decoratively onto cookies.