Add your vanilla (or other flavoring) to your warm water and set aside.
Using a stand mixer with a paddle attachment or a hand mixer on low, mix your powdered sugar, meringue powder, and cocoa powder together.
With your mixer on low, slowly pour your warm water with your flavoring into the dry ingredients.
As soon as all of the ingredients are incorporated, scrape down the bottom and sides of your bowl. It will look super dark. It will lighten once mixed.
Turn your mixer to medium-high and beat until the royal icing can hold a soft peak.
This is a fairly thick consistency because that's how I prefer mine. Even when flooding my cookies, I use it thicker than most people. Remember, the thinner the royal icing is, the longer it will take to dry. You can very easily add small amounts of water to thin it out to your preferred consistency.
To store, place into an airtight container. I also put a piece of plastic wrap onto the royal icing to keep it from forming a crust. I prefer to keep mine in the fridge for up to a month (although, it's usually gone within 2 weeks).