In a large pot, add the cubed potatoes, garlic, and add cold water to cover the potatoes and 1-inch above the potatoes.: The water should start cold so the potatoes heat evenly from the inside out, which yields a tender center without a mealy exterior. As the pot warms, you will hear the faint whisper of small bubbles before a steady simmer, and the garlic will begin to release a subtle aroma into the water. If you use hot water, the exterior will cook much faster than the interior, leading to uneven doneness. A common mistake is overcrowding the pot, which lengthens cooking time and increases the chance of uneven cooking; arrange the potatoes so they sit in one layer as much as possible.
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.: Salting the cooking water seasons the potatoes through and helps the starches gel properly. Listen for the water to reach a rolling boil, then reduce to a steady boil. You'll know they are ready when a fork glides into a chunk with minimal resistance and the edges begin to fray slightly. Overcooking can make them waterlogged, while undercooking leaves hard centers. If your fork meets resistance, continue cooking and test again every few minutes.
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.: After draining, returning the potatoes to the warm pot helps evaporate surface moisture so the mash will be more absorbent and not watery. You will notice a faint sizzling and the steam will rise, indicating moisture is leaving. Keep the heat low so you do not brown the potatoes. A common pitfall is leaving them wet; skipping this evaporating step often results in thin, gluey mashed potatoes.
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.: Warming the dairy melds the fats and allows them to be absorbed smoothly into the potato starch. You should see the butter fully melted and the milk steaming but not boiling, and you will smell a toasty dairy note. Scalded milk gives off a slightly burned scent and can alter the final flavor, so keep the heat gentle. If you forget to warm the liquids and add them cold, the mash cools and may develop a gummy texture.
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.: As you begin mashing, you will hear soft crushing sounds and see the potatoes transform from chunk to fluffy mass. A potato ricer creates the lightest texture with minimal effort, while a masher gives more control. If you use a mixer, pulse gently to avoid overworking the starches. Overmixing introduces too much air and activates the starch into a gluey consistency. Watch for a consistency that holds shape but still yields easily under pressure.
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.: Once you fold in the warm dairy, the potatoes will shine and the texture will smooth out, releasing a comforting aroma of butter and garlic. The cream cheese will add a velvety richness and the sour cream a bright, tangy lift. Folding gently preserves the fluffy structure while ensuring even distribution of fat. If you add too much liquid at once you may overshoot the texture, so take it gradually. Serve while warm; cooled mashed potatoes tend to firm up and lose that just made creaminess.
