Pastry chefs use egg washes on baked goods such as cutout cookies mostly to lend shine, according to "Fine Cooking" magazine, but the egg yolk also produces a golden hue. If you want your cookies to have more visual interest or to complement the color scheme of a particular occasion, add food coloring to your egg wash. Decorative sugar sprinkled on top of the wash makes for a dramatic sparkle.
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Crack open an egg, place the contents into a bowl, and discarding the shell.
Beat the egg with a fork until the yolk and white are mixed well and the liquid is a uniform color.
Add 1 tsp. water to the egg mixture and beat again. If you want to tint your egg wash, add food coloring drop by drop until you achieve the desired color.
Brush the mixture onto the tops of your cookies with a pastry brush. Sprinkle the cookies with colored or turbinado sugar if you wish.
Bake your cookies as usual.