Partially boil yams in large pot (do not remove skin)
When yams are cooked through but not mushy remove from the stove, dump the hot water and cover yams with cold water and enough ice to cool them off (as much as you can fit in the pot is good).
When cool enough to handle, remove the skins (they should peel off easily) and slice yams into 1/2 inch thick pieces and layer them in a casserole dish. Between layers and again on top of the last layer, add dots of butter and sprinkle brown sugar. After the last layer has been dotted with butter and sprinkled with brown sugar, drizzle the maple syrup over all and top with marshmallows.
Bake in 350 degree oven for 30 to 45 minutes or until marshmallows are golden brown.