Sift the powdered sugar into a large bowl to remove any lumps. Measure out the meringue powder, warm water, vanilla, almond extract (if using), and salt (if using).

In the bowl of a stand mixer (or a large mixing bowl for a hand mixer), whisk together the sifted powdered sugar and meringue powder on low speed for a few seconds to distribute the meringue powder evenly.

With the mixer on low, slowly add 6 tablespoons of warm water along with the vanilla and any other extracts you are using. Mix on low until the mixture looks thick and clumpy, then begins to smooth out, scraping down the sides of the bowl as needed.

Increase the mixer speed to medium-high and beat for 5–7 minutes (closer to 7 minutes with a hand mixer), until the icing is thick, glossy, and holds stiff peaks that stand upright when the beater is lifted. If the icing is still droopy, beat longer or add 2–3 tablespoons more powdered sugar.

For outlines and details, transfer some of the stiff icing to a bowl. Add warm water 1/2 teaspoon at a time, stirring gently with a spatula, until the icing flows smoothly but still holds its shape. A line drawn through the icing should very slowly melt back together in about 15–20 seconds.

For flooding cookies, place some icing in another bowl. Add warm water a tiny bit at a time (start with 1/2 teaspoon), stirring gently, until a ribbon of icing dropped from a spoon smooths out in about 10–12 seconds. If the icing becomes too thin and runny, thicken it by stirring in powdered sugar a tablespoon at a time.

If you’d like extra shine and a slightly softer bite, stir 1–2 teaspoons of light corn syrup into the prepared icing until fully combined.

Divide the icing into separate bowls for each color and consistency you need. Keep any icing you’re not working with covered with plastic wrap touching the surface to prevent crusting. Add gel food coloring a few drops at a time, stirring until you reach the desired shade, stopping just shy of the final color since it will deepen as it sits.

Transfer the icing to piping bags fitted with small round tips (or use tipless bags). Decorate cookies or gingerbread as desired. Let the decorated cookies dry at room temperature for 6–12 hours, or overnight, until the surface is fully dry and no longer tacky before stacking or packaging.

For short-term use, keep icing covered at room temperature with plastic wrap touching the surface for up to 8 hours, or refrigerate in an airtight container (with plastic wrap on the surface) for up to 1 week. For longer storage, freeze in airtight containers or zip-top bags (labeling color and consistency) for up to 1 month. Thaw overnight in the refrigerator, bring to room temperature, then stir gently and adjust with a little water or powdered sugar if needed.