Place potatoes into a large pot, cover with water, and bring to a boil over high heat. Reduce heat to medium-low and simmer until tender, about 20 to 25 minutes. Drain and return potatoes to the pot, stirring gently over medium heat until dry (1-2 minutes).

Mash potatoes with a hand mixer until smooth. Stir in cream cheese, half-and-half, chives, egg whites, onion powder, salt, and pepper until well blended.

Spray a slow cooker insert with nonstick spray, add mashed potatoes, cut 1/2 cup butter into 8 pieces, and scatter over the potatoes. Allow potatoes to cool, then cover with the lid and refrigerate overnight.

Remove potatoes from the refrigerator and let them come to room temperature (about 3 1/2 hours).

Place a towel under the lid to absorb condensation and cook on Low heat for 2 1/2 hours while stirring once or twice.