Royal Icing sounds like what you are looking for. Here's a great site that has 2 recipes:
http://www.joyofbaking.com/RoyalIcing.html
Hard Cookie Icing
2 egg whites
2 1/2 cups powdered sugar
1/4 teaspoon cream of tartar
Optional:
1/2 teaspoon extract (vanilla, almond, peppermint, lemon, etc.)
Food coloring
Beat whites until soft peaks form. Slowly add the powdered sugar and the cream of tartar. Add extract. If more than one color is needed, divide into bowls and stir in coloring. Cover bowls with damp towels to prevent icing from drying out. Ice cookies and allow icing to harden. Icing can be piped onto cookies with decorating bag or gun.
This one is from Wilton:
Hard Cookie Icing
It dries to a shiny, hard finish and is also great for outline and fill-in decorations.
1 cup powdered sugar, sifted
2 teaspoons milk
2 teaspoons light corn syrup
Place sugar and milk in bowl. Stir until mixed thoroughly. Add corn syrup and mix well. For filling in areas, use thinned icing (add small amounts of light corn syrup until desired consistency is reached).
You can dip the top of the cookie into the icing and set on a rack to firm up. Before they dry you can pipe in dots or lines or swirls of a contrasting color and it will sink into the icing. Or wait till the main coat dries and pipe decoratively on top with the same icing and it will dry too, but on top of the icing. Put sprinkles or colored sugar on the icing before it hardens.
This one from Landolakes.com has great reviews:
Powdered Sugar Glaze
This almost translucent glaze firms up after standing and gives cut-out cookies a smooth glazed surface -- perfect for decorating cookies.
2 1/2 cups powdered sugar
2 tablespoons water
1 tablespoon LAND O LAKES Butter, softened
1 tablespoon light corn syrup
1/2 teaspoon almond extract or vanilla, if desired
Food color, if desired
Combine powdered sugar, water, butter, corn syrup and vanilla in small mixer bowl; mix until powdered sugar is moistened. Beat at medium speed until smooth, adding additional water if necessary to reach desired spreading consistency. Tint with food color, if desired. Glaze cooled cookies. Let stand until hardened (6 hours or overnight). Yield: 1 1/3 cups