Peel, chop, and soak the potatoes. Peel the Russet potatoes and cut them into evenly sized 1 1/2 to 2-inch chunks. As you work, place the cut potatoes in a large bowl or pot of cold water. This helps prevent discoloration and rinses away some excess surface starch while you finish peeling and chopping the remaining potatoes.
Cook and steam-dry the potatoes. Drain the soaking water, then transfer the potatoes to a large stockpot and cover with fresh cold water by about 1 inch. Season the water generously with salt (about 1 tablespoon) and bring to a boil over high heat. Reduce to a gentle simmer and cook until the potatoes are fork-tender, about 15 to 20 minutes, depending on the size of the pieces. Drain well, then return the potatoes to the warm pot for 1 to 2 minutes to allow excess moisture to steam away.
Mash the potatoes. Using a potato masher, mash the warm potatoes until most of the larger pieces are gone. Do not overwork them.
Warm the dairy. Meanwhile, combine the salted butter, whole milk, sour cream, and black pepper in a small saucepan over low to medium-low heat. Warm gently, stirring occasionally, until the butter has melted and the mixture is warm. Do not boil.
Add the dairy and mix. Pour the warm dairy mixture over the mashed potatoes. Gently stir or fold just until everything is combined and creamy. Avoid overmixing.
Taste, adjust, and serve. Taste and add more warm milk, butter, sour cream, salt, or black pepper as needed. Transfer to a serving bowl and top with additional butter, chopped parsley, and freshly ground black pepper, if desired.