Smooth mixture into a greased 9x12 (or similar) casserole dish. Bake for 25 minutes.
Remove from oven and place pecans then marshmallows on top. You may not need to use the whole bag of marshmallows but use enough to cover the whole casserole. Return to oven for about 5-7 minutes or until toasted on top. Watch carefully to ensure the marshmallows don't burn.