Place chicken breasts in a bowl or gallon freezer bag and pour buttermilk over them. Cover and refrigerate for at least 4 hours or overnight.
Season the chicken breasts with salt and black pepper, if desired. In a shallow dish, combine flour, garlic powder, onion powder, paprika, salt, and black pepper.
Remove chicken from the buttermilk, allowing excess to drip off. Coat each chicken breast in the seasoned flour mixture, pressing the breading onto the chicken.
In a large skillet, heat vegetable oil to350° F (175°C). Carefully place breaded chicken breasts in the hot oil and fry until golden brown and cooked through (about 6-8 minutes per side).
Transfer to a paper towel-lined plate to drain excess oil. If you want to serve as fried chicken sandwiches, brush the brioche buns with butter, if desired. Toast brioche buns lightly.
Place fried chicken on the bottom half of each bun. Add your favorite toppings, such as lettuce, tomato, pickles, and mayo. Top with the remaining bun halves and serve immediately.