1. Heat oven to 350 degrees F. In a bowl, combine masa harina and salt. Pour 2 cups boiling water over masa harina; stir well. Mix in melted butter. Cover dough with plastic wrap and let rest for 15 minutes.
2. Shape dough into 8 balls. Place each ball between 2 pieces of plastic wrap and crefully flatten with a pan to form disks measuring 3 inches wide and 1/2 inch thick. Mend any cracked edges with your fingers. Place arepas on a baking sheet; cover with plastic wrap.
3. Heat 1 Tbs. oil in a cast-iron skillet or saute pan over medium heat. Add 4 arepas to skillet. Cook 5 minutes on each side, until golden brown. Transfer to a baking sheet. Add remaining 1 tbs. oil to skillet and cook last 4 arepas. Place baking sheet with all 8 arepas in oven and bake at 350 degrees F for 15 minutes.
4. Meanwhile, slice chorizo lengthwise, then cut each of those pieces in half on the bias (for a total of 8 pieces). In the same skillet, cook chorizo over medium heat until hot and browned, about 6 minutes.
5. Slice arepas lengthwise about three-fourths of the way through. Fill each with a piece of chorizo, avocado, and cheese. Drizzle with tomatillo salsa, if desired, and serve.