Season chicken pieces with salt, pepper, paprika, garlic powder, onion powder, and chili powder if using.
Pour buttermilk over the chicken and toss to coat, then marinate for 15 to 30 minutes.
Mix flour and cornstarch in a shallow dish and lightly season with salt and pepper.
Heat oil in a deep skillet or pot to about ¾ inch depth and heat to 350°F.
Remove chicken from buttermilk, letting excess drip off, then dredge in flour mixture pressing to adhere.
Fry chicken in batches for 3 to 5 minutes until golden and crispy, then drain on paper towels.
Warm corn tortillas in a dry skillet or microwave until soft and pliable.
Assemble tacos by placing fried chicken pieces in tortillas and topping with lettuce, tomatoes, avocado, and cilantro.
Add optional sauces or squeeze lime wedges over tacos before serving.