Prepare cake batter according to package directions. Pour into a greased and waxed paper-lined 13-in. x 9-in. baking pan. In a small bowl, beat the cream cheese, sour cream, sugar, egg and vanilla until smooth. Drop by tablespoonfuls about 1 in. apart onto batter.
Bake at 350° for 40-45 minutes or until center is firm when lightly touched. Cool for 10 minutes before removing from pan to a wire rack to cool completely.
To make Frankenstein, cut a piece of cake for the head (about 10 in. x 7-3/4 in.) and neck (about 2-1/2 in. x 4-1/2 in.). Cut two pieces for ears (about 2-3/4 in. x 1/2 in.). Save remaining cake for another use. Position head, neck and ears on a covered board.
Place 1/4 cup whipped topping in a small bowl; tint dark green with moss green food coloring. Cut a small hole in the corner of a small plastic bag; insert round pastry tip #3 and fill with tinted topping. Set aside.
Tint remaining whipped topping moss green. Frost face, neck and ears, building up areas for the forehead, nose and cheeks. With reserved dark green topping, pipe mouth, stitches on neck and forehead, and eyes with pupils.
Cut a hole in the corner of a pastry or plastic bag; insert pastry tip #233. Fill with chocolate frosting; pipe hair 1-1/4 in. from top of head to forehead and about 4 in. down sides of head. Cut cake roll in half widthwise; place on each side of neck for bolts. Store in the refrigerator.