Let the cookies cool completely before icing and decorating.
Icing
In a bowl, whisk together the powdered sugar and meringue powder.
Add the vanilla and water to the powdered sugar mixture, and whisk until smooth and drizzly.
To ice the cookies, hold a cookie by its edges and dip the top of the cookie in the icing. Lightly shake off the excess icing, then set it right side up to dry. If the icing gets too thick to dip the cookies as you're working, you can add more water, a teaspoon at a time.
Decorating Tips
When decorating cookies iced with royal icing, there are a few best practices to follow for the most beautiful results.
When whisking the icing, try not to beat too much air into it, which will result in air bubbles on your iced cookies.
After the icing has set for 20-30 seconds, use a toothpick to pop any tiny air bubbles that rise to the surface of the iced cookies, and to smooth out any uneven areas.
If decorating the cookies with sprinkles or sparkling sugar, add these within about 30-60 seconds, before the icing sets and dries. If you add sprinkles and sugar immediately, they will sink into the icing. But if you wait too long, and the icing has fully dried, they'll sit on top of the icing and won't stick.
Notes
Storing the Baked Cookies: The baked cookies should be cooled completely then stored in an airtight container for up to 5 days. If you won’t be frosting and decorating them immediately, freeze the cookies in an airtight container for 3-6 months.Storing the Iced Cookies: Iced cookies should be dried completely for at least several hours, before storing in a single layer in an airtight container. Note that the color on the sprinkles will likely bleed into the icing by the next day - unfortunately I don't know of any way to prevent this. When just making these to enjoy at home, of course, my kids don't care how the cookies look. But for a more professional finish, consider making a thicker batch of red royal icing to pipe the hearts onto the cookies, instead of using sprinkles.
Keyword Christmas Cookies, Cutout Cookies, Grinch, High Altitude