1teaspoonvanilla extractyou may use other flavorings instead/in addition, but make sure they are not made with oil or they can keep your icing from reaching the proper consistency.
NOTE: Your bowl and all utensils must be clean and grease free in order for the icing to whip properly.
In the bowl of a stand mixer fitted with a whisk attachment or in a large bowl (you will need an electric hand mixer) combine sugar, meringue powder, and water. Beat on low speed until combined, then add vanilla extract (or other flavoring).
Gradually increase mixer speed to high and beat until icing forms stiff peaks (5 minutes or longer).¹
Divide and color icing as desired (I like to use gel food coloring).
Icing at this consistency can be used as a stiff outlining icing. To thin the icing for flooding/filling, tint as desired first (so that your outlining and flooding icing is the same tint), divide (so you have one bowl of outlining and one bowl of flooding icing), and then stir your bowl of flooding icing, adding about a teaspoon of water at a time until frosting reaches desired consistency. For me this usually means that if I pull my spoon out of the royal icing, the icing that drizzles off holds its shape for several seconds before dissolving back into a puddle.
To Decorate Cookies:
First pipe your outlines with your outlining frosting, then fill with your flooding icing. Use the point of a toothpick to swirl/smooth the icing until it is smooth and even.
Allow cookies to dry completely before stacking/transporting, this will take several hours. I like to let mine dry overnight to ensure they are dried all the way through.
¹Personally I rarely do intricate designs on my cookies. If you are doing simple decorations, I recommend simply beating the ingredients together until they reach a consistency where the icing that drizzles off of your whisk holds its shape for several seconds (5 or so) before dissolving into a puddle. For me it’s often unnecessary to whip all the way to stiff peaks simply to then thin all of my icing back down to a looser consistency.
How to Store Royal Icing:
Royal Icing can be stored at room temperature or in the refrigerator. Transfer frosting to an airtight container and cover with a piece of plastic wrap that’s directly in contact with the surface of the icing before sealing your container. This icing will keep at least several days, but may need to be stirred again before using.
My Favorite "Equipment" for Making Royal Icing
Gel food coloring. A little goes a long way. I love this kit and use it all the time, all of the colors used in the cookies in this post were from the kit I linked to.
Piping Tips. I like using Wilton sizes 2, 3, and 4, they seem to be the easiest to work with and I have multiple of each size. The kit I linked to is a good starting kit that I use all the time but has a lot more tips than you will likely need
Scribing Tools. Toothpicks will work perfectly fine and these are NOT necessary, but I really enjoy using them anyway!
Help! The next day my cookies were splotchy!
I have had this happen to me just once but it did not make me happy! Here is a great tutorial I found for fixing splotchy royal icing. Essentially all you need to do is turn your oven light on (don't actually turn the heat of your oven on, the heat from the light will be sufficient) and let the cookies sit in the oven overnight. Worked like magic for me, though the cookies did end up being much crisper after a night in the oven (but at least they were beautiful!).