In a large saucepan over medium heat, combine the plantains and stock or broth.

Cook for 15 minutes, or until the plantains are very tender. Drain in a strainer, reserving the cooking liquid. Set aside.

Heat the oil and butter in the saucepan. Add the onions, garlic, tomatoes and cook for 3 minutes, or until onions are tender.

Place the plantains and beans in a food processor. Add the reserved stock, coconut milk, brown sugar, cilantro, cumin, and tomato mixture. Puree and season with salt and pepper. Place in a pot a cook stirring often for 5 minutes.

Transfer to a serving bowl.