Line cookie sheet with waxed paper. Place sandwich cookies on cookie sheet. In resealable food storage plastic bag, place frosting and food colors; seal bag. Squeeze bag until frosting is well blended. Cut small opening in bottom corner of bag.
Pipe thick line across each sandwich cookie near the top. Pipe 1 large dot of frosting above line on each cookie. Pipe squiggles of frosting to resemble witch's hair below each green line. Pipe 2 dots for eyes, 1 for nose and 1 for mouth on each cookie. Place baking bits on dots for eyes and mouth; place candy corn on dot for nose.
Cut candy bars crosswise into quarters. Cut each quarter in half to form 2 triangles. Place 1 candy triangle over large dot of frosting on each cookie to resemble top of witch's hat.
With serrated knife and sawing motion, cut each chocolate wafer cookie in half. Place cookie half upright on green line at base of each candy triangle to resemble hat brim.