Select ripe plantains with skin partially black. Peel, make 3 diagonal gashes on each side, and soak for 30 minutes in water to cover, using 1 tablespoon salt for each quart (4 cups) of water. Remove and drain well.
Rub plantains generously with oleomargarine and place in a greased shallow baking dish. Sprinkle with sugar and dot with butter.
Cook in preheated oven to 350°F for 30 minutes.
Turn, sprinkle again with sugar and dot with butter, and bake about 30 minutes, or until fork-tender.