Preheat the oven: Preheat the oven to 400°F and line a baking sheet with foil for easy cleanup. Scrub the sweet potatoes and pat them dry.

Pierce and bake: Pierce each sweet potato a few times with a fork and set them on the lined sheet. Bake until fork-tender, about 45 to 50 minutes, with the skins wrinkled and juices bubbling at the ends.

Cool slightly: Let the potatoes rest for about 10 minutes until cool enough to handle. This also firms up the flesh so it scoops more cleanly.

Halve and scoop: Slice each potato in half lengthwise and scoop the flesh into a bowl, leaving about a quarter inch of flesh inside the skins so the shells hold their shape.

Toast the walnuts: While the flesh cools, toast the chopped walnuts in a dry skillet over medium heat for 2 to 3 minutes, stirring often, until fragrant.

Mash the filling: Add butter, maple syrup, milk, cinnamon, nutmeg, and salt to the scooped sweet potato flesh. Mash with a potato masher or fork until smooth and pale orange.

Taste and adjust: Taste the filling and add a touch more maple syrup if you'd like it sweeter, or more milk if it feels thick.

Refill the shells: Spoon the filling back into the potato skins, mounding it slightly higher than the edges.

Add the topping: Sprinkle the stuffed potatoes with toasted walnuts, brown sugar, and a light pinch of cinnamon.

Bake again: Lower the oven to 375°F and bake the stuffed potatoes for 15 to 20 minutes, until the edges caramelize and the walnuts smell toasted.

Cool slightly and serve: Let the potatoes cool for 5 minutes before serving so the filling sets slightly and holds its shape when scooped.