In a large pot, add the cubed potatoes, garlic, and add cold water to cover the potatoes and 1-inch above the potatoes.
Add about 1 Tablespoon of salt to the pot and bring the water to a boil. Boil potatoes about 10-15 minutes or until potatoes are fork tender.
Drain potatoes and add back to the pot. Turn the heat to low and heat the potatoes for 1-2 minutes to get the excess moisture out.
While potatoes are cooking, prepare the butter mixture. In small saucepan or microwave safe bowl, heat the butter, milk, and 1 teaspoon salt until warm and the butter is melted. Do not overheat or the milk will scald. Set aside.
Add the potatoes to a large bowl or stand mixer (or just mash them in the pot they cooked in). Mash the potatoes with a potato masher, potato ricer, or hand mixer being careful not to overmix or they will get gummy.
Add the warm butter mixture and fold in. Fold in the cream cheese and sour cream. Add more salt and pepper to taste. Feel free to add more milk if you want the potatoes thinner. Serve warm with pats of butter on top and chives if desired.
Notes
MAKE AHEAD INSTRUCTIONS
Make mashed potatoes as instructed and transfer to a greased 9x13 baking dish. Cool completely and store covered in the refrigerator for up to 2 days. To reheat, let sit on the counter while oven preheats to 350°F. Bake the mashed potatoes, covered, for 30 minutes. Uncover and bake for another 10–15 minutes, until heated through.