• Peel and cut the potatoes into large 2-inch pieces. Place in a large pot and fill the pot so that the potatoes are covered. Bring to a boil and add plenty of salt (about 1 tablesppon) and let boil until they are very fork tender, 15 to 20 minutes. You want them very tender and easy to mash!
• Meanwhile, heat a large skillet over medium heat and add the bacon and cook until crisp, 3 to 4 minutes per side. Transfer to a paper towel lined plate and set aside.
• Remove all but about 2 tablespoons of the bacon fat from the skillet. Reduce the heat to medium-low heat then add 4 tablespoons of the butter and let melt. Once melted, add the diced onion and garlic and cook, stirring, until very tender. Transfer to a very large mixing bowl (large enough to fit all the potatoes in later).
• When the potatoes are fork tender, drain well then add them to the large mixing bowl with the garlic and onion. Add the stick of butter, sour cream, cream cheese, salt, and pepper. Let sit for a second to allow the butter to begin to melt. Use a potato masher, mash the potatoes until smooth, stirring in between mashing every so often so that the ingredients are well combined. Mash until there are no large clumps.
• Add the milk ½ cup at a time, stirring, until well combined and the potatoes start to absorb the liquid. Continue until all the milk has been added and it is well combined with the potatoes. They will feel thinner and soupier than a traditional mashed potato!
• Grease a 9x13 baking dish, then pour the potato mixture into the baking dish and spread into an even layer.
• Chop up the bacon into small bits and sprinkle evenly over the top of the potato mixture. Then add the parmesan cheese, cheddar cheese and finish with the chives.

If Baking Immediately:

• bake at 350°F, uncovered, for 30-45 minutes. Or until the top is melty, golden brown.

If Making in Advance and Baking Later:

• If making a day ahead, cover the baking dish tightly with foil and refrigerate. About 3 hours prior to baking, remove the potatoes from the fridge to let come to room temperature*. Then bake at 350°F, uncovered, for 30 to 40 minutes, or until the top is melty, golden brown.
• Remove from the oven and let rest for 5 to 10 more minutes so that the potatoes can settle and cool a bit.