Return cooked potatoes to pot and stir over medium heat about 1 minute, until excess water has evaporated. Remove from heat and, using potato masher, mash in hot milk, butter, salt and pepper. Beat with wooden spoon until potatoes are smooth and creamy. Stir in optional add-ins if desired. Create stencils of ghost shapes out of heavy-duty aluminum foil.
Place stencils on cookie sheet or microwaveable serving dish.
Lift stencils off finished ghosts. Using small paring knife, slice olives to create circular shapes to be used for eyes and mouth. Makes 4 to 6 servings.