In 3-quart saucepan, cover potatoes and squash with water; add 1 teaspoon of salt. Bring to boil over high heat; reduce heat, cover and cook until tender, 12 to 15 minutes.
Meanwhile, add 2 tablespoons of the butter and the sage to a small skillet or saucepan over medium heat. Tilting pan and watching closely, cook about 3 minutes, until butter foams and begins to brown; keep warm. Thoroughly drain potatoes and squash, return to pan and shake 1 to 2 minutes over low heat. Roughly mash with hand masher leaving mixture chunky. Over low heat, gently mix in remaining tablespoon butter and enough milk for consistency desired. Season with salt and pepper. Spoon into serving bowl and drizzle with brown butter and sage.