Trace a 6×10-in. skull shape on parchment paper. Coat area with cooking spray; set aside.
Melt butter in a large saucepan over medium-low. Add marshmallows, and cook, stirring constantly with a rubber spatula, until completely melted, 2 to 3 minutes. Remove from heat, and add cereal. Stir with a rubber spatula until cereal is evenly coated in marshmallows. Transfer cereal mixture to prepared parchment paper.
Coat palms of your hands with oil. Press cereal mixture in an even layer on parchment paper, following the skull shape. It should be about 1 1/4 in. thick.
Use the back of a large metal spoon to make indentations for the eyes, nose, and mouth. Decorate with Starbursts, chocolate pieces, and fruit rolls as shown, or make your own design by gently pressing candy into cereal mixture.
Transfer to a serving platter. Slice and serve.