Preheat oven to 375°F and butter a 9×13-inch baking dish. Prick sweet potatoes with a fork and place on a sheet pan.
Roast until skins crack and flesh is tender, 45–55 minutes. Let cool slightly.
Slice open potatoes and scoop flesh into a bowl. Add milk, butter, maple syrup, and salt. Mash until mostly smooth.
Pecan Streusel Topping
Reduce oven to 350°F. In a bowl, combine pecans, brown sugar, flour, and salt. Cut in butter until crumbly.
Spread sweet potato mixture in dish and sprinkle topping evenly.
Bake until topping is golden and set, about 30 minutes. Let rest 10 minutes before serving.
Notes
During prep I discovered pricking and oiling tubers yields a deeper caramel aroma.My cooking note: chilling the streusel for 5 minutes prevents it from sinking into the mash.Storage tip: refrigerate leftovers up to 4 days, or freeze sealed for 2 months.