In a small bowl, stir 20 drops of green food coloring into vanilla frosting. Place chocolate sprinkles in a small bowl. For each head, trim off one rounded end of an cookie. Frost the top and sides, then dip the straight end into the sprinkles. Press a few more sprinkles onto the sides of the head for a rounded hairline. Halve a black licorice nib for the bolts. Press them in place, along with mini chocolate chip eyes and a chocolate sprinkle mouth.