Preheat your oven to 350°F (175°C). Make sure the oven rack is positioned in the middle.
Bring a large pot of water to a boil. Carefully add the sweet potatoes and cook for 20 to 25 minutes until very soft and tender.
Drain the sweet potatoes well and let them sit on a paper towel to absorb excess water.
While the potatoes cool slightly, melt the butter in a pot or microwave. Add the brown sugar, flour, and cinnamon. Stir until slightly clumpy.
In a large bowl, mash the sweet potatoes with melted butter, salt, allspice, brown sugar, and eggs until smooth and creamy.
Transfer the sweet potato mixture into a 9x13 inch baking dish and spread evenly.
Sprinkle the pecan topping evenly over the surface.
Top with marshmallows and bake for about 30 minutes until topping is golden and marshmallows are lightly melted.
Allow the casserole to cool slightly before serving.