Roast the sweet potatoes: Preheat your oven to 400°F (200°C). Prick sweet potatoes all over with a fork, place on a foil-lined baking sheet, and roast for 50 to 60 minutes until completely fork-tender and caramelized syrup oozes from skins. Let cool until easy to handle, then peel away skins (they slip off effortlessly).

Whip the fluffy sweet potato base: Lower oven temperature to 350°F (175°C). In a large mixing bowl, combine warm roasted sweet potatoes, melted butter, pure maple syrup, heavy cream, beaten eggs, vanilla, cinnamon, nutmeg, and sea salt. Beat with an electric hand mixer on medium speed for 2 minutes until light, creamy, and completely lump-free.

Spread into casserole dish: Grease a 9x13-inch ceramic baking dish with butter. Transfer the whipped sweet potato mixture into the dish, smoothing the top with an offset spatula into an even layer.

Prepare crunchy pecan streusel: In a medium bowl, whisk together the dark brown sugar, flour, and cinnamon. Add the cold cubed butter and use a pastry cutter or clean fingertips to work the butter into the flour until crumbly pea-sized clusters form. Fold in the chopped pecans.

Top and bake: Scatter the pecan streusel evenly across the sweet potato filling. Bake at 350°F (175°C) for 35 minutes until the casserole is puffed, bubbling around the edges, and the pecan topping is fragrant and golden-brown.

Optional marshmallow crown: If using marshmallows, scatter mini marshmallows across the hot streusel and return to the oven for 3 to 5 minutes (or under a low broiler for 90 seconds) until toasted golden-brown and puffed.

Rest and serve: Let the casserole rest on a wire rack for 10 minutes to allow the filling to set cleanly. Scoop warm portions and serve as the star side dish at your holiday table!