Royal Icing

4 cups powdered sugar, sifted
2 Tbsp meringue powder
5 Tbsp water
additional water as needed

In the bowl of a stand mixer, use the paddle attachment to mix together all ingredients.  Let mix for 7-10 minutes, until the sheen has disappeared.  The icing is now at it's stiffest consistency.  To thin it out, continue to add a little bit of water (by teaspoons) and stir by hand until it's at the right consistency for piping.  Separate the icing into different airtight containers, to match the number of colors you plan to use.  Color appropriately with desired gel food coloring.  Transfer only the amount of icing needed for piping to pastry bags fitted with Wilton size 1, 2 or 3 tips.  Pipe outlines on the cookies where you plan to fill the thinner icing, in the next step.  Let set. Meanwhile, keep the additional icing covered to avoid hardening. 

To create the filling, thin out the icing remaining in the airtight containers by very gradually adding teaspoons of water and stirring by hand.  The icing is at the right consistency when it rapidly drips off the spoon and immediately smooths back into the icing remaining in the bowl.  Use either a squeeze bottle or spoon to pour the filler icing into each outlined cookie.  Spread out to the edges with a toothpick until smooth.  Pop any bubbles with the toothpick, as well.

Let set completely for a few hours or overnight, and then decorate with additional piping icing as desired.

Source: Adapted from Annie's Eats