Sift 4 cups powdered sugar
Beat 2 large egg whites or 3 tablespoons meringue powder with 4 tablespoons water until foamy
Add the powdered sugar gradually while mixing on low speed
Add 1 teaspoon lemon juice or vanilla extract if desired
Beat on medium-high speed until stiff, glossy peaks form
Adjust consistency with small amounts of water for thinner icing
Adjust consistency with more powdered sugar for thicker icing
Cover the icing with plastic wrap or a damp towel to prevent drying
Use immediately or store airtight in the refrigerator