Preheat the oven to 400 degrees F. Coat the bottom of a 13 by 9 by 2-inch glass baking dish with 2 tablespoons butter. Set aside
In a heavy medium pot add the yams and cover with cold water. Bring to a boil over high heat. Reduce the heat and simmer until tender, about 10 minutes. (The color of the yams will turn brighter).
Meanwhile, put the crackers, almonds, cinnamon, and salt in a food processor and pulse a few times to combine until coarse but not fully ground. Add 6 tablespoons butter and pulse until the mixture forms lumps slightly larger than peas. Refrigerate the streusel topping until ready to use.
Spread half of the bananas and yams in 1 layer in the bottom of the prepared dish. Sprinkle with half of the streusel and half of the marshmallows. Repeat to make 1 more layer. Bake until the marshmallows are golden, about 25 minutes. Serve warm.