Peel the Yukon Gold potatoes and cut them into evenly sized chunks, about 1 to 2 inches.

Place the potato chunks in a large pot and cover with cold water. Add 1 teaspoon of salt to the water.

Bring the pot to a boil over high heat, then reduce to a simmer. Cook the potatoes for 15-20 minutes, or until they are fork-tender.

While the potatoes are cooking, heat the milk and butter together in a small saucepan over low heat until the butter is melted. If using garlic, add it to the mixture and let it infuse for a few minutes.

Drain the cooked potatoes in a colander and return them to the pot. Allow the potatoes to steam for a minute to remove excess moisture.

Using a potato masher or ricer, mash the potatoes until smooth and fluffy. Avoid over-mashing to prevent a gluey texture.

Pour the warmed milk and butter mixture over the mashed potatoes. Add the sour cream, black pepper, and additional salt to taste. Stir until everything is well combined and creamy.

Fold in the grated Parmesan cheese and chopped chives for an extra layer of flavor. Adjust seasoning as needed.