1Heat water and butter to boiling in 5-quart Dutch oven; remove from heat.
2Stir in milk and 4 pouches potatoes just until moistened. Let stand about 1 minute or until liquid is absorbed. Whip with fork until smooth.
3Spray inside of 3 1/2-quart slow cooker with cooking spray. Spoon mashed potatoes into slow cooker; keep warm over low heat. Serve with toppings.