For the icing:
Combine the powdered sugar, meringue powder, and water in the bowl of a stand mixer fitted with the paddle attachment. Mix on low speed until the sheen has disappeared and the icing has a matte appearance (about 5 minutes). This will be the stiffest consistency of the icing, and at this point it is still too stiff to use for decorating. Add water a very small amount at a time and stir each addition until fully incorporated. Continue until the icing has reached a consistency appropriate for piping. (If you aren’t sure whether your icing is too thick or thin, test out a small amount. If you are having any difficulty piping, it is still too thick. Add a little more liquid and try again – you do not want to be struggling with icing that is too thick so don’t be afraid to add a little more water when necessary.)
Once you have reached the appropriate piping consistency, tint your icing if desired with gel coloring. I prefer Americolor brand gels because they seem more concentrated and require far less color than other brands I have tried. Whatever you do, if you are aiming for a pastel or light color, add the coloring a very small amount at a time. You can always add more but you can’t take it away.
Using a pastry bag, pipe a border around the edges of each cookie. (I like a Wilton #2 small round tip for edging.) Let stand so the icing will set. Make sure to keep any icing not in use covered at all times so that it does not begin to harden.
Any time that a bag of icing is not in use, stand upright, tip down, in a very small amount of water. This prevents the icing in the tip of the pastry bag from hardening up and clogging the tip.
Once all the cookies have been edged, transfer some of the remaining icing to a separate bowl or container. Thin out by incorporating a small amount of water at a time, until the icing drips off the spoon easily when lifted and then smooths in with that still in the bowl. Did you go too far and now the icing is too thin? Good news! If you over-thin the icing, stir in a bit more powdered sugar to thicken it again.
When the icing has been thinned to the desired consistency, let stand about 5 minutes to allow the air incorporated during the thinning process to bubble up to the surface. Give the icing a gentle stir, popping the bubbles (but not incorporating new ones!)
Transfer the thinned icing to a squeeze bottle (or a plastic bag with a hole in one corner), and flood the area surrounded by the piping on each cookie. If it does not completely spread to the edges on its own, use a toothpick to help it along. If any small air bubbles arise at this point, you can pop them with a toothpick and they should disappear. Allow to set at least 8 hours or overnight, uncovered.
Use the remaining thicker icing for piping decoration as desired. If you make a mistake, it’s okay! Just wipe it off with a clean towel or paper towel and try again. The matte flooded surface will be unharmed. Let dry at least 1 hour before packaging.
A few frequently asked questions, answered:
How far in advance can the cookies be made?
Sugar cookies (at least this recipe) decorated with royal icing tend to keep very well when stored airtight at room temperature. The royal icing helps to seal in the moisture of the cookie. I typically make and decorate cookies up to a week in advance of when I plan to serve them.
Can the cookies be frozen at some point in the process?
The cookie dough can be frozen and used at a later time. Additionally, the cut and baked undecorated cookies can be frozen. However, once decorated, cookies with royal icing should not be refrigerated or frozen. Condensation can form on the icing during the thaw process and cause flaws in the icing design.
Do you cover the cookies when you leave them out to dry overnight?
No, covering the cookies has the potential to mess up the pretty design you worked so hard on. The cookies won’t dry out in the course of one night.
How much icing will I need to decorate a batch of cookies?
This is a tough question because it depends on many factors, including the size of the cookies, the number of colors of icing needed, and complexity of design. Even after years of decorating, I am not great at estimating this. Thankfully it only takes a few minutes to mix up extra icing.