Peel the potatoes completely and cut into even 1.5-inch chunks. Uniform pieces ensure they cook at the same rate.
Place potato pieces in a large pot and cover with cold water by at least 1 inch. Add 1 tablespoon of salt to the water.
Bring to a boil over medium-high heat, then reduce to a gentle simmer. Cook until a fork easily pierces through with no resistance, about 15-20 minutes.
Drain the cooked potatoes in a colander, then return them to the hot pot. Allow them to sit for 2 minutes, gently shaking occasionally to allow excess moisture to evaporate.
While still hot, mash the potatoes using a traditional potato masher or ricer (for the smoothest results). Avoid food processors or blenders, which can create a gluey consistency.
Add the room temperature butter first, mashing until melted and incorporated.
Gradually add the warmed milk or cream, stirring gently with a wooden spoon until the desired consistency is reached.
Add salt, pepper, and any optional ingredients (sour cream, roasted garlic). Taste and adjust seasonings as needed.
Serve hot, garnished with fresh chives if desired.