Preheat the oven to 350 degrees. Pierce each sweet potato several times with a fork and place in a baking pan or on a rimmed baking sheet. Bake until very tender when pierced with a fork, about 1 hour 15 minutes. Remove from the oven and let rest until cool enough to handle, about 20 minutes.
Meanwhile, in a small saucepan, melt the butter over medium heat; stir in the maple syrup. Bring to a boil, then remove from the heat and cover with foil to keep warm.
Cut the sweet potatoes in half and scoop the flesh into a large bowl, discarding the skins; mash with a potato masher. Stir in the warm butter mixture and milk. Mash to combine; season with salt and pepper.
Transfer the mashed sweet potatoes to a microwave-safe dish or serving bowl and cover with foil. Just before serving, remove the foil and microwave on high power in 2-minute increments, stirring in between, until heated through.