Peel the russet potatoes and cut them into even chunks, about 1-2 inches in size. This ensures they cook uniformly.
Place the potato chunks in a large pot and cover them with cold water. Add 1 teaspoon of salt to the water to enhance the flavor of the potatoes.
Bring the pot to a boil over medium-high heat, then reduce the heat to medium-low and simmer until the potatoes are fork-tender, about 15-20 minutes.
While the potatoes are cooking, gently heat the milk and butter together in a small saucepan over low heat until the butter is melted and the mixture is warm. This helps to keep the potatoes hot when mixing.
Drain the cooked potatoes thoroughly in a colander and return them to the pot. Allow them to steam for a minute to evaporate excess moisture.
Using a potato masher or ricer, mash the potatoes until smooth. Avoid over-mashing, as this can lead to a gummy texture.
Gradually pour in the warm milk and butter mixture while continuing to mash until you reach your desired creaminess.
Season the mashed potatoes with black pepper and garlic powder, if using. Mix in sour cream for added richness, if desired.
Taste and adjust seasoning as needed. Serve immediately, garnished with chopped chives if desired.