Melt butter in a saucepan over medium heat. Add brown sugar and pecans. Simmer for 3 minutes. Meanwhile, place drained yams in a large bowl and mash finely with a potato masher (leaving some chunked pieces). Pour sugar/pecan mixture over yams. Stir until combined thoroughly. Add in cinnamon and nutmeg. Transfer to a metal pie pan. Top with marshmallow topping. Bake for 15 minutes at 325 degrees F. Remove from the oven and raise the oven temperature to 400 degrees F. Bake for an additional 10 minutes at 400 degrees F to brown the top, watching carefully to keep from over-browning.