Preheat oven to 350F.

Place the potatoes in a large saucepan and cover with cold water. Bring to a boil, then reduce to a simmer and cook until the potatoes are fork tender, about 10 to 15 minutes. Drain well.

Meanwhile, melt butter in a small saucepan over medium heat. Reduce to low and cook until butter starts to turn light brown.

In a large bowl, mash the potatoes with a potato masher and fold in brown butter, milk, and ground cinnamon until just incorporated. Season with salt to taste.

Pour sweet potato mixture into a buttered casserole dish and spread into an even layer. Top with marshmallows and pecans, then bake for 20 to 25 minutes until the top is golden brown.

Allow to cool for 10 minutes before serving.