Prep the potatoes: Peel the Yukon Gold potatoes and cut them into even 2-inch chunks. Place them in a large pot and cover with cold water by about 2 inches. Add enough salt so the water tastes like the sea (about 2 tablespoons).
Cook the potatoes: Bring the pot to a boil over high heat, then reduce to a steady simmer. Cook for 15-20 minutes until the potatoes are completely tender. A fork should slide through with no resistance.
Warm the dairy: While the potatoes cook, combine the butter, milk, and heavy cream in a small saucepan over low heat. Warm until the butter melts and the mixture is steaming. Do not boil. Keep warm.
Drain and dry: Drain the potatoes thoroughly in a colander. Return them to the hot, empty pot and let them sit for 1-2 minutes to steam off excess moisture.
Mash: Mash the potatoes using a potato ricer (for smoothest results) or a hand masher. Do not use a food processor or blender.
Add the butter: Add the warm butter to the mashed potatoes and fold gently until fully incorporated.
Add the milk mixture: Gradually pour in the warm milk and cream mixture, folding gently after each addition, until you reach your desired consistency. You may not need all of it.
Season and serve: Stir in the kosher salt and white pepper. Taste and adjust seasoning as needed. Transfer to a serving bowl, top with an extra pat of butter and chives if desired, and serve immediately.