Step 1: In a large mixing bowl (preferably of a stand mixer fitted with a whisk attachment), combine the 1/2 cup warm water and meringue powder. Whisk on medium speed for 1 minute until frothy.
Step 2: Reduce mixer speed to low. Gradually add the sifted powdered sugar, one cup at a time, mixing well after each addition until fully incorporated. Add the vanilla extract.
Step 3: Once all sugar is added, increase mixer speed to medium-high and beat for 5-7 minutes, until the icing is bright white, glossy, and holds stiff peaks. Do not overmix.
Step 4: At this point, you have stiff consistency royal icing, perfect for piping details or borders. For a medium consistency (good for outlining and some flooding), gradually add 1-2 tablespoons of additional water, mixing slowly until desired consistency is reached (it should run off a spoon in a thick ribbon).
Step 5: For a flooding consistency (thinner, for filling in large areas), continue adding water, 1 teaspoon at a time, mixing slowly. The icing should flow smoothly and self-level within 5-10 seconds after a spoonful is dropped back into the bowl. This adjustment is key for a flawless royal icing recipe.
Step 6: If coloring, separate the icing into desired portions and add gel food coloring a tiny drop at a time, mixing thoroughly. Keep any unused royal icing covered tightly with plastic wrap pressed directly onto the surface to prevent crusting.