Next, stir together black beans, egg, crumbs, cilantro, lime zest, chili powder, salt, pepper, and cayenne pepper. Stir together until well mixed, and add the zucchini mixture. Mix until evenly combined, and divide into 6 equal balls. Place onto an oiled baking sheet (I line mine with aluminum foil), and smash into patties about 1 inch thick. Place patties in the oven and bake for 8-10 minutes. Remove from the oven and flip. Bake for another 8-10 minutes, or until cooked through and firm. Divide onto buns with cheese, avocado, onion, spinach, mustard, and mayo, if desired. ENJOY!