Royal icing is a sweet, hard icing that's made from egg whites and confectioners' sugar. It's the perfect icing for decorating cookies, and you can use all kinds of colors to achieve pretty much any effect you can imagine.
It's best to use liquid pasteurized egg whites for making royal icing, but you can substitute powdered egg whites. Just reconstitute the egg whites according to the instructions on the package, enough to equal four egg whites.
Besides adding flavor, the lemon juice also helps harden the icing, but you could substitute vanilla extract if you prefer.
A decorating kit with a piping bag and various tips are certainly useful, or you can use a squeeze bottle. But you can also cut out a triangle of ordinary parchment paper, wrap it into a cone and use that.
Just spoon some of the icing into the wide end of the cone, fold the top over, and then squeeze the icing out the pointy end. You can snip off the tip of the cone to adjust the size of the opening. (Note, however, that this old-school trick is more difficult if you're trying to use parchment paper that's been treated with a nonstick coating — which is more or less the norm these days.)
- 1 lb powdered sugar (about 4 cups or 450 grams)
- 4 egg whites (or 1/2 cup liquid egg whites)
- 1 teaspoon lemon juice
- Optional: food coloring in desired colors
Gather the ingredients.
In the bowl of a stand mixer fitted with the whip attachment, beat the egg whites on high speed until foamy.
Switch to low speed and gradually sift the sugar into the egg whites until it's completely incorporated.
Then add the lemon juice and beat on high speed until the icing is very thick and forms stiff peaks, about 5 to 10 minutes.
At this point, you can add the food coloring, if you're only going to need a full batch of one color. But if you need more than one color, separate whatever quantities of icing you need into individual bowls and just stir in the coloring you want.
Now you can use royal icing to decorate your cookies.
You can thin the icing out with water, depending on how thick you want it. A good technique is to pipe the borders with the straight icing, and then thin it out to fill in the centers. Watch the video I linked to above for a demonstration of what I mean.
You should use the royal icing right away or store it in an airtight container because it will harden quickly when exposed to the air. You can store it for a few days in the fridge, as long as it's in an airtight container.