1 recipe Royal icing (see additional recipe below)
In a large mixing bowl, beat butter and shortening with an electric mixer for 30 seconds. Add sugar, baking powder, and salt; beat until combined. Beat in egg, milk, and vanilla until combined.
Beat in as much of the flour as you can with the mixer. Stir in any remaining flour. Cover; chill dough about 1 hour or until easy to handle.
Preheat oven to 375 degrees F. On a lightly floured surface, roll a portion of the dough at a time to 1/4-inch thickness. Using start-shape cookie cutters, cut out six 4-1/2-inch stars, six 3-3/4-inch stars, four 2-3/4-inch stars, four 2-inch stars, and six 1-1/2-inch stars.
Place larger stars about 1 inch apart on one ungreased cookie sheet and smaller sizes on another ungreased cookie sheet. Bake for 8 to 10 minutes or until the edges are firm and bottoms are very lightly browned.
Cool on cookie sheet for 1 minute. Transfer to a wire rack, cool.
To assemble each cookie tree, place one of the large star cookies on a plate. Spread a small circle of royal icing onto center of cookie; top with two more of the large cookies, rotating each slightly (spread icing between cookies as you stack them).
Continue stacking with three 3-3/4-inch stars, two 2-3/4-inch stars, two 2-inch stars, and three 1-1/2-inch stars.
To decorate, pipe a small amount of royal icing down one side of tree, letting icing drip slightly from tips; sprinkle with edible glitter. Repeat on other sides of tree.
Stir together powdered sugar, meringue powder, and cream of tartar. Add the 1/4 cup warm water. Beat with an electric mixer on low speed until combined. Beat on high speed for 7 to 10 minutes until very stiff. Add the 1 to 2 tablespoons additional water, 1 teaspoon at a time, to make desired consistency.