1Heat oven to 350°F (325°F for dark and nonstick pan). Grease bottom only of 9-inch square pan with shortening or cooking spray. (For easier cutting, line pan with foil, then grease foil on bottom only of pan.) Make brownies as directed on box. Cool completely, about 1 1/2 hours. Cut into 4 rows by 3 rows to make 12 brownies.
2Divide frosting between 2 small bowls, placing a little more frosting in one bowl. Add green food color to the bowl with more frosting and purple food color to second bowl; mix both until blended. Frost 8 brownies with about 1 tablespoon green frosting; frost remaining brownies with purple frosting.
3Decorate each brownie with 2 candy corns for neck "bolts" and 2 candy eyeballs. Use decorating gel to draw mouth and hair.