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.