Prepare sugar-cookie dough, divide into two disks, and refrigerate 1 hour. Heat oven to 350°F. Shape level tablespoons of dough into balls. Using your hands, roll each ball into a 5-inch log; flare one end. Place cookies 1 inch apart on baking sheets.
Bake cookies until golden, about 20 minutes. Transfer to wire rack and cool.
To decorate, tint 1 cup royal icing green, transfer to resealable bag, and snip corner. Thin remaining icing with 1 tablespoon water. Brush tops of pipes with white icing, and sprinkle with glitter. Pipe green line down center of each cookie. Place M&M's on lines. Pipe brown icing spirals on flared ends.
In large bowl, with electric mixer on low, beat together powdered sugar, powdered egg whites, water, and cream of tartar until well combined. Increase speed to high, and continue beating until stiff peaks form. If not using immediately, press piece of plastic wrap directly onto surface of icing and store in refrigerator for up to 1 week. Bring icing to room temperature before using.