Boil the plantains in salted water about 20 minutes, until fork-tender. Drain, cool, and peel.
Mash the plantains with butter, salt, and cornstarch until smooth.
Flatten a portion of the mash, add a spoonful of mozzarella, and fold to seal. Repeat to form 8 to 10 balls.
Heat oil to 350°F (175°C). Fry the plantain balls in batches 3 to 4 minutes, until golden. Drain.
Cook the prosciutto slices in a dry skillet 3 to 4 minutes, until crisp. Crumble once cool.
Melt the butter, add the onion, cook 3 to 4 minutes. Add garlic, cook 1 minute.
Stir in the flour and cook 1 minute. Gradually whisk in the broth, then add the potatoes. Simmer 12 to 15 minutes.
Stir in the cream and half the prosciutto. Season with salt and pepper.
Ladle into bowls, top with a plantain ball or two, remaining prosciutto, and parsley.