In a mixer, combine meringue powder, light corn syrup, vanilla extract, and water.
Using a hand whisk, mix ingredients until the mixture becomes foamy.
Add the powdered sugar to the mixture.
Hand whisk the mixture until the powdered sugar is incorporated and looks soupy.
Using the paddle attachment on a stand mixer, beat mixture on medium-high speed for about 5-7 minutes. The mixture should begin to thicken and whiten. You should see stiff peaks form when you lift the attachment. This icing consistency is good for piping.
NOTE: to thicken royal icing, add powdered sugar. To lessen the thickness (for flooding), add water 1/2 teaspoon at a time until you reach your desired consistency. Flood icing should flow somewhat like honey. When you drip it off a spoon, it should lay on top of the rest of the icing for about 10-15 seconds before disappearing. People refer to flood icing in different terms. You may hear, “flood icing” or “10 second icing” or “15 second icing”. Depending on how fast you count, people have different terms, but they all basically mean the same thing.