Heat the oven and prepare the dish. Heat the oven to 375 F (190 C). Lightly grease a 9x13-inch baking dish with butter.

Boil the sweet potatoes. Put the sweet potato chunks in a large pot, cover with cold water and add a pinch of salt. Bring to a boil and cook for 20 minutes, until the potatoes are very tender when pierced with a fork. Drain very well and return them to the hot pot for 1 minute so the last steam escapes.

Make the streusel while the potatoes cook. In a small bowl, stir together the chopped pecans, brown sugar, flour, melted butter and cinnamon until the mixture holds together in damp crumbs. Set aside. If you are using the marshmallow topping instead, skip this step.

Mash the sweet potatoes. Transfer the drained potatoes to a large bowl. Add the melted butter, brown sugar, eggs, milk, vanilla, cinnamon, nutmeg and salt. Mash with a potato masher or beat with a hand mixer on low speed until smooth, with a few small lumps left for texture. Taste and add a pinch more salt if needed.

Top and bake. Spread the mashed sweet potatoes evenly in the prepared baking dish. Scatter the pecan streusel over the top. Bake uncovered for 25 minutes, until the filling is hot all the way through and the topping is golden and crisp. For the marshmallow version, bake the casserole plain for 20 minutes, then scatter the marshmallows over the top and bake 5 minutes more, until puffed and lightly golden.

Rest and serve. Let the casserole rest for 5 minutes before serving so the filling sets and each spoonful holds together. Serve hot.