In a medium bowl, using a whisk, or hand mixer on low, mix together powdered sugar, meringue powder, corn syrup, and vanilla extract.
Gradually add water, about 2–3 tsp for piping, or 6–7 tsp for dipping.
Beat on medium until smooth and glossy with a spatula, or hand mixer.
Divide icing, as needed, if adding gel food coloring.
Pipe, flood, or dip cookies immediately.
Cover any unused icing tightly to prevent drying.
Easy Hack: Dip tops of cookies, push icing to edges with a scribe needle in a swirl motion, then gently shake upright.