In a stand mixer fitted with the whisk attachment (or in a large bowl with a hand-held electric mixer), whisk the powdered egg whites and 6 Tbs. warm water on low speed to combine. Then let stand, whisking several times over the course of 5 minutes, until the powder absorbs the water and is dissolved. Increase the mixer speed to medium and beat until frothy, about 1 minute. Add the confectioners’ sugar and mix on low speed until blended, about 1 minute. Increase the speed to high and beat until the mixture is thick and glossy, 3 to 5 minutes. If it seems too thin, add more sugar, a little at a time.
At this point, you can color and/or flavor the icing. To make several colors, divide the icing into small batches in separate containers. Use a toothpick or cake tester to add a small amount of gel food coloring to the icing, stirring until smooth and streak free, and adding more color as needed. Add the flavoring of your choice, if using, and stir until well combined.
To outline a cookie: Spoon some of the icing into a pastry bag fitted with a small plain tip. Practice first on a piece of cardboard or waxed paper; if the icing is too thick to pipe evenly, put it back in the bowl and stir in water, a drop or 2 at a time, until it pipes easily but retains its shape. Outline the cookie with the icing.
At this point, decorate the outline further while it's still wet with your choice of topping (see below), or if you want to fill in the outline completely with icing (see flooding, below) set the outlined cookies aside to dry until tacky, about an hour, depending on the humidity of your kitchen.
To coat an entire cookie with icing (flooding): Thin some of the icing by stirring in cold water, 1 tsp. at a time, until just pourable; you don't want flooding icing to hold its shape but rather to fill in and cover an area on the cookie, so the consistency should be like melted chocolate. Spoon the flooding icing into a pastry bag fitted with a medium plain tip. Pipe a tight zigzag of flooding icing inside the border of an outlined cookie and then use a toothpick to spread the icing in an even layer out to the edges.
If you're adorning the cookies with additional decorative toppings, such as dragées or sanding sugar, add
the toppings while the icing is still wet. Let sit for about 10 minutes,
then invert the cookie to remove any excess topping.
When the cookie is completely decorated, set it aside to dry completely, 2-1/2 to 4 hours.
To pack iced cookies: Pack each cookie in individual cellophane envelopes and layer between sheets of thin foam in a cookie tin or box.
If not using the icing immediately, put a damp paper towel directly on the surface to prevent a skin from forming; you can let the icing sit at room temperature for up to 2 hours. If not using within 2 hours, cover the bowl with plastic wrap and refrigerate for up to 12 hours. Let the icing come back to room temperature before using. If the icing is stiff, add a few drops of water and beat briefly on low speed to loosen.