Wash and scrub the potatoes. Bake at 400 degrees F for about 1 hour.
Cut off the tops lengthwise and scoop out the insides. Mash and season. Add milk and beat until fluffy. Fold in egg whites. Refill shells with potato mixture. Place on a baking sheet that has been lightly sprayed with oil.
Brown in the oven at 400 degrees F for about 25 minutes.
NOTE: Top with shredded cheese if desired. If made ahead and refrigerated, add 10 minutes baking time.