Preheat the oven to 350°F and set an oven rack in the middle position. Line a baking sheet with foil for easy clean-up.
Prick each sweet potato 3 to 4 times with a fork. Place on the prepared baking sheet and bake until tender, 60 to 75 minutes; turn the potatoes over halfway through baking.
Meanwhile, melt the butter in a medium saucepan over medium heat. Add the maple syrup, milk, salt and pepper. Turn off the heat and set aside.
When the sweet potatoes are cooked and cool enough to handle, cut them in half. Holding them in one hand with an oven mitt, scoop the warm flesh out into the pan with the butter mixture. Over low heat, mash the potatoes with a potato masher or fork until smooth. Taste and adjust seasoning if necessary. Transfer to a bowl and sprinkle with fresh thyme (if using). Serve hot.
Note: These sweet potatoes can be made entirely ahead of time. Before serving, stir in a bit of milk and reheat on the stovetop or in the microwave.
