This royal icing with meringue powder is the recipe I use for decorating sugar cookies. It dries smooth and firm enough to stack, but still has a soft bite when you eat it.
Sift the powdered sugar: Sift the powdered sugar into the bowl of your stand mixer.
4 cups powdered sugar | 480g
Add the meringue powder: Add the meringue powder and whisk by hand until evenly combined with the powdered sugar.
3 tablespoons meringue powder | 7g
Add water: Pour in the water.
7 tablespoons water
Mix by hand first: Whisk until the sugar, meringue powder, and water are combined. This helps prevent dry clumps at the bottom of the bowl and keeps powdered sugar from flying when the mixer starts. Shake off any excess from the whisk.
Mix on medium speed: Attach the whisk attachment and mix on medium speed (about 6 on a KitchenAid) for 4 minutes.
Scrape the bowl: Stop the mixer and scrape down the sides and bottom of the bowl with a spatula.
Finish mixing: Continue mixing on medium speed for another 5 minutes, until stiff peaks form and the icing leaves defined lines in the bowl.
Store or use: Transfer the icing to an airtight container. Use as a base to color or adjust consistency, or refrigerate until ready to decorate.