Line a 12-inch round pizza pan with foil; grease foil.
Prepare brownie mix as directed on package, using the water, oil and egg; spread in foil-lined pan.
Bake for 30 to 34 minutes; do not overbake; cool completely.
Frost brownie with vanilla frosting.
Make a spider web by piping the decorating icing in a spiral pattern, using the round tip; start the spiral pattern at the outside and keep the lines about ¾ inch apart; using a toothpick or knife, draw lines outward through frosting starting at center of brownie.
To make spider, cut gumdrop crosswise into thirds; place smaller rounded end on brownie, rounded side up, to form body of spider; cut 4 thin strips from each remaining gumdrop slice to form legs; Place next to spider body.