Peel and dice the potatoes. Place the potatoes in a pot of salted water and bring to boil. Cook until the potatoes are tender. Drain and then return the potatoes to the pot. Add the butter and milk and mash well. Allow the potatoes to cool completely to room temperature before continuing.
In a mixing bowl, combine the cooled mashed potatoes and the eggs. Lightly whisk to ensure the eggs are fully mixed in but don't overwork the potatoes or they could become gluey. Add the shredded Parmesan, mozzarella, and a half cup of the cheddar along with the chives, kosher salt, and pepper. Stir until combined.
Preheat the oven to 400° F. Spray a muffin tin with non-stick spray. Sprinkle each muffin cup with grated Parmesan and tilt the pan so that each cup is coated. Spoon the potato mixture into the muffin cups, filling them three-quarters full. Top with the remaining cheddar cheese.
Bake the puffs for 30 to 35 minutes until they are well-browned. Let cool briefly in the muffin pan, no more than 5 minutes. Carefully remove the potato puffs and transfer to a rack.
Serve the potato puffs warm with sour cream and chives to garnish. Enjoy!
Notes
*Alternatively, use 3 cups prepared, cooled mashed potatoes. Omit the butter and milk as well.