In a large bowl, combine the masarepa, salt, and sugar. Add the room-temperature butter and 1 ½ cups of shredded mozzarella. Mix thoroughly so everything is evenly incorporated. This step ensures the dough has consistent flavor and texture.
Pour in the milk and slowly add lukewarm water while mixing. Knead until the dough is smooth and soft, adding a little extra water if needed. Let the dough rest for 5 minutes to allow it to absorb the liquid fully.
Divide the dough into 6 equal portions and roll each into a ball. Flatten each ball into a disc about ½ inch thick. Make sure the edges are smooth for even cooking.
Heat a nonstick skillet over medium heat and lightly spray with cooking spray. Cook each arepa for about 5 minutes per side until golden brown. Medium heat allows the cheese to melt without burning the outside.
Let the arepas cool slightly before serving. This helps the cheese inside set just enough so it doesn’t spill out immediately when you bite into it.