Royal Icing
There is a great FAQ about this icing at the Bake at 350 website linked below.

Ingredients
- (This will cover 2-3 dozen 3.5 inch cookies in 2 colors; I usually double this recipe.)
- 4 TBSP meringue powder
- scant 1/2 c. water
- 1 lb. powdered sugar
- 1/2 - 1 tsp light corn syrup
- few drops clear extract (optional)
Directions
- Combine the meringue powder and water. With the paddle attachment of an electric mixer, beat until combined and foamy.
- Sift in the powdered sugar and beat on low to combine. (Do NOT skip the sifting!)
- Add in the corn syrup and extract if desired.
- Increase speed to med-high/high and beat for about 5 minutes, just until the icing is glossy and stiff peaks for.
- (You should be able to remove the beater from the mixer and hold up and jiggle without the peak falling.) Do not overbeat.
- Cover with plastic wrap touching the icing or divide and color using gel paste food colorings.
Original source: bakeat350.blogspot.ca