Prepare the potatoes: Peel (optional) and cut the Yukon Gold potatoes into even 1- to 1 1/2-inch chunks so they cook uniformly.

Cook until tender: Place the potato chunks in a large pot and cover with cold water by about an inch. Add a generous pinch of salt. Bring to a simmer over medium-high heat and cook until a fork slides through easily, about 12–15 minutes.

Warm the dairy: While potatoes cook, gently warm the milk and butter together in a small saucepan until the butter melts and the mixture is hot but not boiling. If using sour cream, have it ready at room temperature.

Drain and steam dry: Drain the potatoes in a colander and let them sit for a minute to release steam. Return them to the warm pot for 30–60 seconds over low heat to evaporate excess moisture—this helps keep the potatoes fluffy.

Rough mash first: For the smoothest result, use a ricer or a potato masher to break up the potatoes. If you prefer ultra-silky potatoes, pass them through a ricer or food mill into a mixing bowl.

Whip gently: Transfer the potatoes to a large mixing bowl. Using an electric hand mixer on low to medium speed (or a stand mixer with the paddle), begin whipping the potatoes while slowly adding the warm butter-milk mixture. Whip just until smooth and airy—overmixing can make them gluey.

Finish and season: Fold in the sour cream (if using), then season with kosher salt and freshly ground black pepper to taste. If adding garlic, fold in now if roasted or sautéed, or stir in a small amount of garlic-infused oil for subtlety.

Serve immediately: Spoon the whipped mashed potatoes into a warmed serving bowl. Top with a pat of butter, a sprinkle of chives, and an extra grind of black pepper. Enjoy while hot and fluffy.