Preheat oven to 350 degrees F (175 degrees C). Grease a 9x12-inch baking dish.
Mash sweet potatoes in a bowl with milk, adding the milk a little at a time until potatoes are the desired consistency. Spread sweet potatoes into the prepared baking dish.
Bring maple syrup and butter to a boil in a saucepan and pour the boiling syrup mixture over the sweet potatoes.
Bake in the preheated oven until the sweet potatoes are hot and the topping has begun to cook into a sticky caramel, 20 to 30 minutes.