Preheat your oven to 350°F (175°C).

Lightly butter or spray a 9x13-inch baking dish.

Spread pecans on a baking sheet and roast them for 5-7 minutes until fragrant and lightly browned.

Peel and cube sweet potatoes, cover them with water in a large pot, and boil for about 15 minutes until fork-tender. Drain and mash until creamy.

In a bowl, combine softened butter, brown sugar, flour, and a pinch of cinnamon until crumbly.

In a large bowl, stir mashed sweet potatoes with brown sugar, eggs, milk, melted butter, vanilla, cinnamon, nutmeg, and chopped pecans until smooth.

Spread half of the sweet potato mixture evenly in the baking dish, sprinkle with half of the cinnamon filling, then add the remaining sweet potato mixture and cinnamon filling.

Sprinkle remaining pecans over the casserole and add mini marshmallows on top.

Bake for about 30 minutes until golden brown and filling is hot. Cover with foil if marshmallows brown too quickly.

Let it rest for a few minutes before serving warm.