Wash and peel the Yukon Gold potatoes if desired. Cut them into evenly sized chunks (about 1–1.5 inches) so they cook uniformly.
Place the potato chunks in a large pot and cover with cold water by about an inch. Add a generous pinch of salt to the water.
Bring the water to a gentle boil over medium-high heat, then reduce to a simmer. Cook until the potatoes are fork-tender, about 12–15 minutes depending on the size of the pieces.
While the potatoes cook, warm the milk and cream in a small saucepan with the butter until the butter is melted and the mixture is hot but not boiling. If using garlic, add it to the milk mixture to infuse, then remove the garlic before adding.
Drain the potatoes thoroughly in a colander and return them to the warm pot off the heat for a minute to evaporate excess moisture; this helps keep the mash fluffy rather than watery.
Using a potato ricer or masher, work the potatoes until smooth but not overworked. For the silkiest texture, push the potatoes through a ricer directly into the pot or bowl.
Gradually add the warmed milk, cream, and butter mixture to the potatoes, stirring gently with a wooden spoon or spatula until you reach your desired consistency.
Season with salt and freshly ground black pepper, tasting and adjusting as needed. Fold in chopped chives if using.
Serve immediately, topped with an extra pat of butter and a sprinkle of chives, or keep warm in a covered dish until ready to enjoy.