In a large pot of boiling salted water, cook the potatoes until tender, about 25 minutes. Drain the potatoes. Return to the same pot. Stir over medium heat until the excess moisture evaporates, about 1 minute; remove from heat. Add 5 tablespoons of butter to the potatoes; mash well. Add 2/3 cup of milk and mix, adding more milk by tablespoonfuls to reach your desired consistency. Season the potatoes to taste with salt and pepper.
Melt 3 tablespoons of butter in a large skillet over medium-high heat. Add half of the mushrooms; sauté until beginning to soften, about 3 minutes. Add the remaining mushrooms and shallots. Sauté until the juices evaporate and the mushrooms brown, about 12 minutes; season with salt and pepper. (Potatoes and mushrooms can be prepared 2 hours ahead. Let stand separately at room temperature. Re-warm separately over medium heat before continuing.)
Mix 2/3 of the mushrooms into the potatoes. Mound in a serving bowl. Top with the remaining mushrooms; sprinkle with chives.