Combine the masarepa, warm water, and salt in a large bowl. Knead with your hands until a smooth, pliable dough forms without any dry lumps.

Let the dough rest for 5 minutes to allow the cornmeal to fully hydrate; this prevents the cakes from cracking during cooking.

Divide the dough into 8 equal portions and roll them into smooth balls. Flatten each ball between your palms into a disk about 1/2-inch thick. If the edges crack, dampen your fingers with a little water and smooth them out.

Heat the oil in a large nonstick skillet over medium heat until it shimmers. Arrange the disks in the pan and cook until golden brown and crusty on both sides, about 6 to 8 minutes per side.

Mix the filling while the arepas cook. Mash one avocado in a medium bowl with the lime juice, chili powder, salt, and pepper until creamy, then gently fold in the remaining chopped avocado, tomatoes, bell pepper, scallions, and cilantro.

Split the warm arepas halfway through with a paring knife to create a deep pocket. Stuff generously with the avocado mixture and serve immediately while the corn cakes are still hot and crisp.