In 2-quart saucepan, heat milk, water and margarine to a rapid boil over medium-high heat (watch carefully to avoid boilover).
Remove from heat. Stir in potatoes just until well blended. Let stand 4 to 5 minutes or until liquid is absorbed.
Meanwhile, cut 4 pieces plastic wrap, about 9x12 inches each. Line insides of 4 (6-oz) custard cups with plastic wrap; spray with cooking spray. Divide potatoes evenly among custard cups. Press firmly; turn upside down onto individual serving plates.
Remove plastic wrap from each potato mound; choose from suggested ingredients above to make faces. Serve immediately.