For the wonton cups: Preheat the oven to 375 degrees F.
Brush 12 cups of a mini muffin pan with the butter. Fit 1 wonton wrapper into each cup, folding the sides as needed.
In a small bowl, toss together the beans, green chiles, chili powder and lime juice. Spoon some of the bean mixture into each cup, about a heaping tablespoon each. Bake until the filling is warmed through, about 12 minutes. Remove the pan and top each cup with a pinch of cheese. Return the pan to the oven and continue to bake until the cheese is melted and the wontons are golden, another 5 minutes.
For the toppings: Let the taco cups cool in the pan for a minute or two. Transfer the taco cups to a platter and top each with some avocado, pico de gallo, a small dollop of sour cream, hot sauce, if using, and cilantro.