Peel the potatoes, and cut them into cubes. Add the potatoes to a large sauce pan and add enough cold water to cover them by a couple inches. Cook the potatoes on medium heat for about 15 minutes or until you can easily pierce them with a fork. Transfer to a colander to drain.
In the meantime chop the bacon slices into small pieces. Cook the bacon until crispy and browned.
Alternatively, you can cook the bacon slices first, then chop them up into small pieces. Place the bacon on paper towels to remove some of the fat.
Using a potato masher or fork, mash the potatoes in a big bowl until fluffy. Add the cream cheese, butter, sour cream, milk, half the cheddar cheese, half the mozzarella cheese and mix until well combined. To make them even fluffier you could use a hand mixer and mix until light and fluffy. Add the chives, half the bacon and salt and pepper. You might also want to reserve some chives for the topping.
Stir everything together and transfer to a buttered baking dish. Top with remaining cheddar, mozzarella, bacon and chives.
Bake for about 30 minutes or until top is golden brown.