Adjust the oven rack to the middle position and heat the oven to 400°F. Whisk the masarepa, salt, and baking powder together in a large bowl, then gradually stir in the water until a smooth dough forms. Divide the dough into eight equal portions and shape each into a 3-inch round, about 1/2 inch thick.

Heat half of the oil in a 12-inch nonstick skillet over medium-high heat — the oil should shimmer but not smoke. Add four arepas and cook until deeply golden on both sides, about 4 minutes per side. Transfer the arepas to a wire rack set over a rimmed baking sheet, wipe out the skillet, and repeat the process with the remaining oil and arepas.

Bake the arepas on the wire rack until they sound hollow when tapped on the bottom, about 10 minutes.

While the arepas bake, mash the black beans in a bowl using a potato masher or fork until most are broken but some texture remains. Stir in the Monterey Jack, cilantro, scallions, lime juice, and chili powder, then season with salt and pepper to taste.

Using a fork, gently split the hot, baked arepas open to create a pocket. Stuff each with a generous portion of the bean filling and serve immediately while the cheese is melting.