In a mixing bowl, combine the masa harina, baking powder, and salt. Cut in the vegetable shortening until the mixture resembles coarse crumbs. Gradually add the 3/4 cup of vegetable broth until a soft dough forms. Set aside.
In a large pot, heat a drizzle of oil over medium heat. Add the diced onion, bell pepper, and minced garlic. Sauté for about 5 minutes until the vegetables are softened.
Stir in the ground cumin and chili powder. Pour in the 4 cups of vegetable broth, along with the black beans and corn. Bring the mixture to a gentle simmer for about 10 minutes to blend the flavors.
While the soup is simmering, take small portions of the masa dough (about 1 tablespoon each) and roll them into balls.
Gently add the dumplings to the simmering soup, making sure they float on the surface. Cover the pot and let them cook for about 15 minutes, or until they have puffed up and are cooked through.
Once the dumplings are done, stir in the lime juice and season with salt and pepper to taste.
Ladle the soup into bowls. Top with fresh cilantro, avocado slices, and sliced jalapeños if desired.
Notes
Feel free to adjust the spice level by adding more jalapeños.