Peel and chop the Yukon Gold potatoes into chunks, ensuring they're evenly sized for consistent cooking. Place them in a large pot filled with cold salted water, bringing it to a gentle boil.

Boil the potatoes for 15–20 minutes until they're fork-tender.

Drain the potatoes carefully, returning them to the pot after draining. Let them sit for a minute.

Mash the potatoes using a masher, ricer, or hand mixer until they’re smooth and creamy.

Mix in the unsalted butter, sour cream, milk or heavy cream, and cream cheese, if using.

Season with salt and black pepper to taste.

Transfer the mashed potatoes into a buttered 9×13-inch baking dish and smooth out the top with a spatula.

Optional: Dot the top with additional butter or sprinkle cheese for an extra indulgent layer.

Bake at 350°F (175°C) for 25–30 minutes until the casserole is heated through and lightly golden.

Garnish with chopped chives or parsley before serving.