Place the potatoes in a pot large enough to accommodate them and cover with water. Cover and bring to a boil. Reduce heat to simmer before it boils over. Cook until they are tender and then drain. If using Russet, place them on a cutting board or dish until they are cool enough to handle. The skin will peel pretty easily. For Yukon Gold, do the same or leave the skin on. Use the same pot the potatoes were boiled in to heat the milk to a simmer. Mash the potatoes with the milk, adding some more milk if needed. Season with salt and pepper to taste. Stir in the chopped watercress. Push in bits of the butter through out so they melt into little pools of yum!