Step 1

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.

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

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.