1. Preheat oven to 350 degrees F. Coat pan with nonstick spray.
2. Prepare brownies as directed on package. Spread batter in pan with spoon. Bake for about 25 minutes, or until a toothpick or fork inserted in center comes out almost clean.
3. While brownies bake, prepare pudding as directed on package.
4. Once brownies are completely cool, spread pudding on top.
5. Decorate. Swamp tree: Tie 4 to 6 gummy or licorice whips together near the ends with thread; twine with skinny whips if desired. Insert tied end in pan. Twist whips down to make a weeping willow tree. Swamp snails: Place peanuts on pudding; use icing to draw eyes and a stripe down back of each. Floating heads: Stick lollipops in swamp. Add faces with tube frosting. Finishing touches: Add spooky gummy creatures; lay pretzels to look like fallen logs; attach lollipop stick to back of cookie with icing. Write "Swamp" on front of cookie with tube frosting.
6. Cut to serve.