Step 1Combine the arepa masa, cheese and salt in a bowl.
Step 2Combine the water, milk and butter in a saucepan and bring to a boil. Remove from heat and cool until just warm enough to touch.
Step 3Add a little of the warm milk mixture to the masa mixture and knead to mix. Add a little more liquid and knead again, continuing until you have incorporated all the liquid and the mixtures have combined to make a dough.
Step 4To make the arepas, take a scant one-fourth cup dough, form into a ball and pat out between the palms of your hands like a tortilla, and to about the same thickness. Or cut a resealable plastic bag in half to make 2 plastic sheets. Place a ball of dough on one plastic sheet, cover with the other sheet and press flat with a rolling pin or a round object such as a flat-bottomed can. The arepas should be about 3 inches in diameter. Place them on a baking sheet and cover with a damp cloth until ready to cook.
Step 5Heat an ungreased seasoned griddle or nonstick skillet and cook the arepas 2 to 3 minutes on each side until lightly golden brown. Serve immediately, with butter if desired.