Preheat the oven to 375°F.

Whisk the masarepa, salt, and baking powder together in a large bowl. Pour in the warm water and stir with a wooden spoon until a soft, uniform dough forms. Let the dough rest for 5 minutes—it should feel slightly firm and easy to handle without sticking to your fingers.

Divide the dough into 8 equal portions and roll them into smooth balls. Gently flatten each ball into a disk about 3 1/2 inches wide and 1/2 inch thick, using a damp finger to smooth any cracks that appear along the edges.

Mash the black beans in a medium bowl using a fork or potato masher until mostly smooth but with some texture remaining. Fold in the cheese, cilantro, scallions, lime juice, and chili powder. Season with salt and pepper to taste.

Heat the vegetable oil in a large nonstick skillet over medium heat until the oil shimmers but does not smoke. Add the dough rounds and cook until the bottoms are deeply golden brown and a distinct crust has formed, about 5 to 7 minutes. Flip and brown the second side, about 5 to 7 minutes more.

Transfer the browned arepas to a wire rack set over a rimmed baking sheet. Bake until they feel firm to the touch and sound hollow when tapped on the bottom.

Let the arepas cool for 5 minutes so they are easier to handle. Using a serrated knife, cut halfway through the side of each arepa to create a deep pocket. Stuff generously with the bean and cheese mixture and serve while the filling is warm and the cheese is melting.