In a medium bowl, whisk together 2 cups confectioners' sugar, 1 large egg, and 1 teaspoon pure vanilla extract. Add 1 tablespoon of water one at a time, whisking until the desired consistency is reached: for lining icing, which is great for dots and patterns like the ones you see on my cookies, you want a soft peak to form when a spoon is lifted from the icing. Whisk in food coloring until homogenous and transfer to a piping bag with a small tip. Use immediately.