How to Make It
In a medium skillet, cook the chorizo over moderate heat, breaking it up with a wooden spoon, until fully cooked, about 5 minutes. Drain the chorizo on paper towels and let cool completely.
In a small saucepan, melt the butter in the milk over moderately high heat. In a large bowl, mix the instant masa with the sugar, salt, mozzarella and cooked chorizo. Add the milk mixture and stir until a smooth dough forms. Cover and let the dough rest for 5 minutes; it should be slightly tacky to the touch.
Roll out the dough between 2 sheets of wax paper to a 1/2-inch thickness. Transfer the dough (in the wax paper) to a baking sheet and refrigerate for at least 30 minutes, until firm.
Preheat the oven to 300°. Peel off the wax paper. Using a lightly greased 2-inch round cookie cutter, stamp out arepitas as close together as possible and transfer them to a baking sheet. Gather up the scraps, roll them between two sheets of wax paper and stamp out more arepitas.
In a large cast-iron skillet, heat 1 teaspoon of the vegetable oil. Add one third of the stuffed arepitas and cook over moderate heat until golden outside and moist within, about 3 minutes per side. Transfer the cooked arepitas to another baking sheet and place in the oven to keep warm. Repeat with the remaining oil and arepitas. Arrange on a platter and serve immediately while hot.
Instant masa, like masa harina, is made from corn treated with limewater. It can be found in the Latin section of most supermarkets or at Latin markets. Do not substitute other types of cornmeal.