Peel the Yukon Gold potatoes and cut them into uniform 1-inch cubes.
Place the cubed potatoes in a large pot and cover with cold water. Add 1 tablespoon of kosher salt.
Bring the water to a boil over high heat, then reduce to a steady simmer. Cook for 15 to 20 minutes until the potatoes are fork-tender.
While the potatoes cook, warm the heavy cream in a small saucepan over low heat. If using garlic, sauté the minced garlic in 1 tablespoon of butter until fragrant and add it to the warming cream.
Drain the potatoes thoroughly in a colander, then return them to the hot pot. Place over low heat for 1 to 2 minutes to evaporate excess moisture.
Add the softened butter to the potatoes and mash with a potato masher or potato ricer until smooth and lump-free.
Gradually pour in the warm heavy cream mixture, folding gently with a wooden spoon or spatula until just incorporated. Do not overmix.
Fold in the sour cream until evenly distributed.
Season with kosher salt and freshly cracked black pepper. Taste and adjust seasoning as needed.
Transfer to a serving bowl, top with an extra pat of butter, and garnish with freshly chopped chives. Serve immediately.
