Heat oven to 350°F (325°F for dark or nonstick pan). Spray 15x10x1-inch pan with cooking spray. Make cake batter as directed on box. Pour batter into pan; spread evenly. Bake 17 to 22 minutes (19 to 26 minutes for dark or nonstick pan) or until toothpick inserted in center comes out clean. Cool completely in pan, about 30 minutes.
Meanwhile, place sheet of foil on cookie sheet. Spoon melted candy melts into small resealable food-storage plastic bag; seal bag. Cut small hole in corner of bag. Squeeze melted candy in shape of tree onto foil. Refrigerate tree. Squeeze melted candy on sandwich cookies to look like tombstones.
In small bowl, mix frosting and food color until blended. Reserve 1/4 cup frosting for roof and decorations. Frost cake with remaining frosting. With sharp knife, cut peaks in one short side of each of 2 graham crackers. Insert crackers into cake for front and back of haunted house. Break 1 graham cracker in half crosswise; insert into cake for sides of house. Frost 2 graham crackers; place on house for roof.
Break up 1 chocolate graham cracker; arrange on cake for walkway and on house for windows and door. Arrange peanut butter pieces along edge of walkway. Pipe line of frosting along top of house; arrange candy corn in frosting.
Arrange chocolate tree on cake. Place candy pumpkins near house. Arrange "tombstones" on cake. Crush remaining chocolate graham cracker; mound graham cracker crumbs in front of tombstones for graves. Store loosely covered.