Microwave potatoes according to package directions.
Stir together potatoes, black beans, and butter in a large microwave-safe bowl. Cover with plastic wrap; fold back a small edge to allow steam to escape. Microwave at HIGH 1 to 2 minutes or until hot. Stir in cilantro. Serve immediately.
Note: For testing purposes only, we used Diner's Choice Country Style Mashed Potatoes. You can substitute 4 cups of your favorite hot mashed potatoes.