Preheat oven to 400*F. Peel plantains. Place on large baking sheet; brush with melted butter. Bake 20 minutes; turn plantains over. Cook 20 minutes longer or until golden browned. If not soft enough, cover plantains with foil; cook 10 to 15 minutes longer or until tender.
In 10-inch skillet over medium heat, in hot oil, cook onion and green pepper about 5 minutes until tender-crisp, stirring occasionally. Remove to plate. In same skillet over medium-high heat, cook ground beef until well browned on all sides, stirring frequently. Add sofrito, cumin, salt and pepper; heat to boiling. Simmer 5 minutes, stirring constantly.
Make a horizontal slit down the center of each plantain, opening up center. Spoon some ground beef mixture into each plantain. Sprinkle with shredded queso blanco. Return to oven; bake 5 minutes longer until cheese is melted.