Step 1

Adjust oven rack to middle position and preheat oven to 325°F. Combine masarepa, cheese (if using), 1 cup water, and 1 teaspoon vegetable oil in a medium bowl and knead with hands until a dough is formed. Take a small amount and flatten it between your palms If the edges crack, knead in more water a tablespoon at a time until dough is supple and smooth but not sticky. Season dough to taste with salt, then cover and set aside for five minutes.

Step 2

Divide dough into four even pieces and roll into balls. Working on a wooden cutting board or a regular cutting board with a sheet of plastic wrap or parchment paper on top of it, flatten each ball down to a disk about 4-inches in diameter and 1/2-inch thick.

Step 3

Melt butter in a 12-inch cast iron or non-stick skillet over medium-low heat (see note #3 above). Add arepas and cook, moving them around the pan and rotating them occasionally, until first side is charred in spots and a dry crust has formed, about five minutes. Flip arepas and cook on second side until a dry crust has formed, about five minutes longer. Transfer to a baking sheet and bake until cooked through, about 10 minutes longer. Remove from oven, let rest 5 minutes, split, fill, and serve.