1. Melt butter in a small saucepan over medium-low heat. Cook, stirring occasionally, 10 minutes or until butter is golden brown and fragrant. Remove from heat.
2. Meanwhile, place potatoes in salted water to cover in an 8-quart soup pot, and bring to a boil over medium-high heat. Reduce heat, and simmer 20 minutes or until tender; drain. Return potatoes to pot, and cook over low heat, stirring occasionally, 1 minute or until potatoes are dry. Remove from heat, and mash potatoes with crème fraîche, salt, and pepper. Stir in browned butter, reserving 1 to 2 tablespoons.
3. Transfer potatoes to a 2-quart ovenproof serving dish. Drizzle with reserved brown butter, and sprinkle with sage leaves. Keep warm until ready to serve.
If making ahead, cover dish with foil, and chill. Remove from refrigerator, and let stand at room temperature 30 minutes. Bake at 350° for 30 to 45 minutes or until heated through.