Preheat the oven to 425°F. Line a baking sheet with foil.

Place the sweet potatoes on the foil and pierce the tops 2 to 3 times with a knife. Loosely cover with foil. Roast until very soft, about 50 to 60 minutes. Uncover and let rest until cool enough to handle. Peel the sweet potatoes, discarding the skin.

In a large bowl, combine the roasted sweet potato, melted butter, heavy cream, light brown sugar, eggs, salt, cinnamon, smoked paprika, and cayenne. Mash until smooth.

Reduce the oven temperature to 375°F. Butter a 13-by-9-inch casserole dish. Place the sweet potato filling in the casserole dish and smooth the top.

In a small bowl, stir together the flour, sugar, cinnamon, and salt. Add the pecans and melted butter, mixing until clumpy.

Sprinkle 1 cup of marshmallows all over the potato filling. Top with the streusel mixture. Bake for 25 minutes.

Remove from the oven and top with the remaining 1 cup of marshmallows. Return to the oven for 10 to 15 minutes, or until the casserole is bubbly around the edges, hot in the middle, and the marshmallows are golden brown.

Let cool for a few minutes before serving.