1Heat broth and rice to boiling 3-quart saucepan; reduce heat. Cover and simmer about 20 minutes or until liquid is absorbed (do not lift cover or stir). Spread rice on ungreased cookie sheet; cool.
2Mix rice, eggs, Parmesan cheese and butter. Shape into 1 1/2-inch balls. Press 1 cube mozzarella cheese, 1 cube prosciutto and 1 cube mushroom in center of each ball; reshape to cover cubes completely. Roll balls in bread crumbs to coat.
3Heat oil (2 inches) in deep fryer or Dutch oven to 375°. Fry 5 or 6 fritters at a time about 2 minutes or until deep golden brown; drain on paper towels.