Heat the oven to 400F. Scrub the sweet potatoes, prick each one a few times with a fork, and set them on a foil-lined baking sheet. Roast 45 to 55 minutes, until a knife slides into the center with no resistance. Let them cool 10 minutes.

Lower the oven to 350F and grease an 8x8 or 2-quart baking dish.

Slip the skins off the sweet potatoes and put the flesh in a large bowl. Mash with a fork or potato masher until mostly smooth; a few small lumps are fine.

Stir in 2 tablespoons of butter while the potatoes are still warm so it melts. Add the milk, egg, maple syrup, vanilla, 1 teaspoon cinnamon, and salt and mix until combined. Spread evenly in the baking dish.

Make the topping. Chop the pecans roughly. In a small bowl, stir together the pecans, oats, whole wheat flour, brown sugar, and 0.25 teaspoon cinnamon. Cut in the cold 1.5 tablespoons of butter with your fingers until the mixture is crumbly and holds together in small clumps.

Scatter the topping evenly over the sweet potatoes.

Bake uncovered 25 to 30 minutes, until the topping is golden and the edges are bubbling. If the pecans brown too fast, tent loosely with foil for the last 10 minutes.

Let it rest 5 minutes before serving.