In a medium bowl, mix together the arepa flour (masarepa), salt, softened butter, and warm water until well combined.
Knead the mixture until a smooth dough forms. Ensure that the dough is not sticky but has a consistency similar to homemade play-dough. Cover the bowl with a kitchen towel and let it rest for a few minutes.
Check the dough's consistency by flattening a small amount between your palms. If the edges crack, the dough is too dry. Add water gradually, around one tablespoon at a time, until the dough becomes the consistency you are looking for.
With lightly moistened hands, divide the dough into 8 equal portions. Roll each portion into a ball, then flatten into circles approximately 5 inches in diameter and ¼ inch thick.
Working with one circle at a time, place a desired filling in the center, leaving a small border around the edge. Top with another dough circle and seal the edges, smoothing them lightly with your palms to form a disk about ½ inch thick. Repeat with the remaining dough.
To cook the arepas on the stove top: Heat oil in a large skillet over medium-low heat. If using butter, heat the skillet first, then add and spread the butter to coat the surface. I recommend adding oil as well to the butter to prevent it of burning.
To cook the arepas on an electric griddle: Preheat the griddle to 325°F/160°C. Add butter and spread it evenly to coat the surface.
Place the prepared arepas on the skillet or griddle and cook, occasionally moving them around, for about 4-5 minutes until golden brown. Flip and continue cooking for another 4-5 minutes until golden brown and crispy. Remove from heat and serve warm. Additional butter on top is optional but highly recommended.