Combine masarepa and salt in a large bowl. Add butter or oil if using. Add warm water gradually, stirring as you go. Use your hands to knead briefly until smooth — about 1 minute. Dough should feel like firm playdough. If it cracks, add water 1 tablespoon at a time. Rest 5 minutes.
Divide dough into 6 equal portions. Roll each into a ball and flatten into a disc about 3/4 inch thick and 3.5–4 inches wide. Smooth any cracks at the edges by wetting your hands lightly.
Heat a non-stick skillet or cast iron griddle over medium heat with a small amount of oil or butter. Add arepas with space between each. Cook 5–6 minutes on the first side without moving until golden brown. Flip and cook another 5–6 minutes. Tap the top — a hollow sound means cooked through.
Let rest 2 minutes. Use a sharp knife to cut around the edge, leaving one side as a hinge, and open like a pita. Fill with your chosen fillings and serve warm.
