Perfect Royal Icing

Ingredients

• 18 ounces organic powdered sugar, divided, plus more if needed
• 2 1/2 ounces egg whites, from 2 large eggs
• 2 teaspoons silver rum or water
• 1 1/2 teaspoons vanilla extract
• 1/2 teaspoon Diamond Crystal kosher salt; for table salt, use half as much by volume or use the same weight
• 1/4 teaspoon cream of tartar
• 1 1/2 ounces heavy cream, plus more if needed
• Gel paste food coloring, optional

Directions

1. To make the icing, add 16 ounces powdered sugar (4 cups; 450g) to the bowl of a stand mixer. Add the egg whites, rum, vanilla, salt, and cream of tartar, then use a flexible spatula to work everything into a smooth paste. Set the bowl over a gently simmering water bath in a 3-quart saucier, stirring until the mixture feels hot when touched, or reaches about 150 degrees F (66 degrees C) on an instant-read thermometer. Move the bowl to the stand mixer and fit it with the paddle attachment. Add the remaining 2 ounces powdered sugar (1/2 cup; 60g), mix on low until combined, then raise the speed to medium and beat until the icing is thick and frosting-like, about 20 seconds, or until the bowl no longer feels warm. Use it right away for thick frosting or dimensional decorations, such as gingerbread buttons or small cake rosettes; tint it, if you like, using the coloring method below. For a looser royal icing for glazing cookies or making more delicate designs, follow the thinning method below.
2. To thin the icing, turn the mixer down to low, pour in the cream, and mix until the icing is smooth. Take the bowl off the mixer, lift the paddle, and let the icing fall back into the bowl while you trace a figure 8. When that figure 8 vanishes in exactly 8 seconds, the icing is the right consistency for glazing cookies. If it disappears more quickly, mix in a spoonful of powdered sugar to tighten it. If it lingers longer than 8 seconds, loosen it with a few drops of cream. Keep testing and adjusting until the figure 8 disappears in exactly 8 seconds.
3. To color the icing, divide it between small bowls, using one bowl for each shade you plan to make. Press plastic wrap directly onto the surface of each portion. Working with one bowl at a time, mix in a few drops of gel paste until the color looks right. Transfer the tinted icing immediately to a disposable pastry bag fitted with a #3 pastry tip, or spoon it into a parchment cone.
4. To glaze the tops of sugar cookies, pipe a border around baked, fully cooled Rolled Sugar Cookie Cutouts, leaving a 1/8- to 1/4-inch edge, or a wider one if the cookies are larger. Drizzle more icing inside the border, then use the piping tip to coax it into an even layer. If you are adding sprinkles or another icing color, move quickly, because royal icing begins to skin over within 90 seconds. For cookies that will stay at home, let the icing dry in the open air until firm, about 4 hours. For cookies you plan to package and ship, let the icing dry until it is hardened all the way through, about 6 to 8 hours more.
5. Cookies decorated with royal icing can be kept at room temperature for up to 1 week; if needed, refrigeration can stretch that to about 10 days. Do not freeze cookies with royal icing, since ice crystals in the icing will thaw into liquid water and leave the surface looking blotchy.