Add hot water, butter, and salt to the bowl. Mix until butter is mostly melted.

Add in masarepa, 1/2 c. at a time. Mix well until fully combined, making sure all of the masarepa is hydrated and consistency of playdoh. It should not be so wet it sticks to your fingers. If it is, add in more masarepa 1 tbsp. at a time.

Portion dough into balls, about 1/4 c. each. It should make about 10 balls.

Indent the ball and stuff with 1 tbsp. mozzarella. Pinch the dough around the mozzarella to close up the hole. Once sealed, roll the ball to smooth the dough. Pat the ball between your hands to flatten, about 1/2" thick. Continue until all balls are stuffed and flattened, moving quickly so dough doesn't dry out.

Heat oil in large skillet on medium heat. Once hot, add the arepas and cook until lightly golden. About 5-7 minutes.

Flip and and cook another 3-5 minutes until golden on both sides.

Remove from pan and allow arepa to drain on plate with paper towel. Sprinkle with salt if desired.

Serve immediately.