Mix together the cornmeal, warm water, salt, and butter. Mix until fully combined, and knead for a few minutes until smooth. Let rest while you cook the beef.
Brown the ground beef with the salt, onion powder, and dried onions. Drain off excess fat, and add in Better Than Bouillon, water, cumin, and chili powder. Simmer over medium to low heat for 15 -20 minutes. Remove mixture from heat and slowly incorporate cheese, mixing frequently, until its fully melted and has tightened up the sauce. Keep warm over low heat while frying your arepas.
Once the beef is being held over low heat, preheat a cast iron pan over medium heat with a teaspoon or two of oil inside. Roll out the arepa dough into balls, press into flat 1/2 inch thick discs, and place into hot pan. Fry 6-7 minutes per side, until golden brown and cooked through. Remove to a paper towel and repeat with the rest of the dough.
Once cooled enough to handle, slice the arepas through the middle, and fill with beef and garnish with avocado, lettuce, and taco sauce. Enjoy hot!