Preheat oven to 375°F (190°C). Lightly butter a 9x13-inch (or deep 2.5-quart) ceramic baking dish. Prick sweet potatoes with a fork and roast on a foil-lined baking sheet for 50 minutes until soft and caramelized. Cool slightly and peel away the skins.

In a large mixing bowl or stand mixer, combine the warm peeled sweet potatoes, browned butter, maple syrup, heavy cream, beaten eggs, vanilla extract, ground cinnamon, nutmeg, and sea salt. Whip on medium speed for 2 minutes until light, creamy, and completely smooth.

Spread the whipped sweet potato mixture evenly into the prepared ceramic baking dish using a spatula, smoothing the top into an even layer.

In a separate medium bowl, make the pecan streusel: whisk together the packed dark brown sugar, flour, and cinnamon. Add the cold cubed butter. Use a pastry blender or your fingertips to work the butter into coarse pea-sized crumbs. Gently fold in the chopped pecans.

Evenly scatter the crunchy pecan streusel crumble across the entire sweet potato base.

Bake at 375°F (190°C) for 25 minutes until the casserole is heated through and the streusel edges are golden, fragrant, and bubbling.

Carefully remove the casserole from the oven. Scatter the mini marshmallows evenly across the top (or over half if your family loves both styles). Return to the oven for 3 to 5 minutes until the marshmallows puff and toast to a deep golden brown. (Watch closely to prevent burning!).

Let the casserole rest for 10 minutes before serving so the spiced sweet potato layer sets and slices cleanly. Serve warm alongside your holiday feast.