How to Make It
Place russet potatoes, Yukon Gold potatoes, stock, 1 teaspoon of the salt, and, if desired, bay leaf in a 6-quart slow cooker. Cover and cook on HIGH until potatoes are very tender, 4 to 5 hours. Drain; let stand 2 minutes. (Remove and discard bay leaf, if using.)
Return potatoes to slow cooker. Add butter pieces, milk, pepper, and remaining 2 1/2 to 3 teaspoons salt to taste. Mash with a potato masher to desired consistency. Serve immediately.