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 let marinate for 15 to 30 minutes.
In a shallow dish, mix flour and cornstarch with a pinch of salt and pepper.
Heat oil in a deep skillet or pot to about 2 cm (3/4 inch) depth and heat to 350°F (180°C).
Remove chicken from buttermilk, letting excess drip off, then dredge in flour and cornstarch 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 in each tortilla, topping with shredded lettuce, tomatoes, avocado, cilantro, and optional sauces or lime wedges.