Mix water, Rice Mix and butter in medium saucepan. Bring to boil. Reduce heat to low; cover and simmer 25 minute or until most of the water is absorbed and rice is tender. Stir occasionally to prevent rice and beans from sticking Remove from heat. Let stand 5 minutes For each tostada shell, top with 1/3 cup rice mixture, ¼ cup chicken and assorted toppings, if desired Cooking tips: Black Beans & Rice Wraps: Use flour tortillas (8-inch) in place of the tostada shells. Serving Suggestion: Top with assorted toppings, such as shredded cheese, shredded lettuce, chopped avocado, salsa, sour cream and chopped fresh cilantro.