Line cookie sheets with parchment paper; set aside. Prepare cookie dough according to recipe directions, chilling if needed for easier handling. Roll out dough between two pieces of parchment paper to 1/4-inch thickness. Remove top layer of parchment paper. Cut dough into 5x3-inch rectangles. Place rectangles 2 inches apart on prepared baking sheets.
Bake in 350 oven for 9 to 11 minutes or until edges are firm and bottoms are golden. Let stand 1 minute on cookie sheet before transferring to wire rack to cool.
For Icing: In a large bowl, stir together powdered sugar, meringue powder and cream of tartar. Add the warm water and vanilla. Beat with an electric mixer on low speed until combined. Beat on high speed for 7 to 10 minutes or until icing is very stiff. If desired, add 1 teaspoon of water at a time to make icing of spreading consistency. To prevent icing from drying out, cover bowls with damp paper towels and plastic wrap.
Spread icing evenly over top of cookie or brush a thin layer over top; let dry. If you like, tint some of the icing and use it to pipe a message. If using edible wafer papers, attach per manufacturer's directions. (We warmed some corn syrup in the microwave and brushed it over the back of the wafer, then placed it on iced side of cookie.) Add additional decoration as you like.
Meringue powder is a mixture of pasteurized dried egg whites, sugar, and edible gums. Look for it in the baking aisle of your supermarket or at a specialty food store.
Search online for Christmas-theme edible wafers if you want to purchase these to decorate your cookies.