Place potatoes into a large pot and cover with salted water; bring to a boil over high heat. Reduce heat to medium-low and simmer until fork tender, 20 to 25 minutes. Drain water and place pot with potatoes back over medium heat until potatoes are dry, 1 to 2 minutes.
Mash potatoes using a hand mixer until smooth. Stir in cream cheese, half-and-half, egg whites, chives, onion powder, salt, and pepper. Blend well.
Spray a slow cooker insert with nonstick spray and add mashed potato mixture. Cut 1/2 cup butter into tablespoon-sized pieces and add to potatoes.
Cook on Low for 2 1/2 hours, stirring once or twice during the cook time. Stir in remaining 1/4 cup of butter 1 tablespoon at a time and cook for an additional 30 minutes.