Cotija is not necessary. Use Cheddar, Moz, Jack, Ricotta, Velveeta, heck even Brie in a pinch. LOL
Here is my favorite Batter for rellenos. Tip: use cold eggs.
For a simple batter, whip 3-6 cold egg whites in a chilled bowl until they are stiff. Stir up the remaining yolks and slowly fold them in with a pinch of salt. Use approximately 2 large eggs for each pound of chiles, or 1 egg per extra-large chile.
If you want to go nuts, here is one of my recipes for Rojo sauce:
- 3 Tablespoons sesame seeds
- 20 Blanched almonds
- 1/2 cup green pumpkin seeds
- 5-6 guajillo Chiles (dried)
- 2-3 New Mexico Chiles (dried)
- 2 Habaneros (dried)
- 2 Tablespoons lard or vegetable oil (lard will make it more flavorful)
- 2/3 cup finely diced white onion
- 3 garlic cloves, roughly chopped
- 1 Tbs Mexican whole Oregano
- 1/2 teaspoon ground cumin
- 3/4 teaspoon ground paprika
- 1/2 teaspoon salt
- 2 pinches ground cinnamon
- 5 cups chicken broth