Bring a large pot of lightly salted water to a boil. Add yams and cook, stirring occasionally, until tender, 12 to 15 minutes.
Preheat the oven to 350 degrees F (175 degrees C).
Drain and transfer to a bowl with butter, 4 tablespoons brown sugar, cream, and 1/2 teaspoon cinnamon. Mash until all ingredients are incorporated and butter is melted. Transfer to a casserole dish and sprinkle with remaining brown sugar and cinnamon. Arrange pecans and marshmallows over the top.
Bake in the preheated oven for 20 minutes.
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