Place potatoes in large pot and add enough water to cover. Bring to boil and cook 13 to 15 minutes, or until very tender. Drain in colander. 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. Working with half to 1 cup mashed potatoes per ghost, use rubber spatula to fill in stencil's ghost shapes. Lift stencils off finished ghosts. Using small paring knife, slice olives to create circular shapes to be used for eyes and mouth.
To warm ghosts, microwave on high (100% power) 3 to 5 minutes on microwavable dish. If using an oven, place potatoes on cookie sheet and reheat at 350 degrees, loosely covered with foil, 10 to 15 minutes or until heated through. Makes 4 to 6 servings.