In a large bowl, whisk together all of the ingredients for the brine. Add the chicken breasts and stir to coat. Cover the bowl with plastic wrap and refrigerate for at least 4 hours, or overnight.
In a separate bowl, whisk together the flour, potato starch, baking powder, garlic powder, onion powder, paprika, cayenne, kosher salt, and black pepper.
Remove the chicken from the brine.
Dredge the chicken breasts in the flour mixture, making sure they are fully coated. Shake off any excess flour.
Dip each chicken breast back into the remaining buttermilk brine and coat with the flour mixture again.
Heat enough vegetable oil in a large, deep skillet or Dutch oven to reach a depth of about 1 inch. Heat the oil over medium-high heat until it reaches 350°F.
Carefully add the chicken breasts to the hot oil, a few at a time, and fry for 6-8 minutes per side, or until golden brown and cooked through.
Use a slotted spoon or tongs to transfer the chicken to a wire rack or paper towel-lined plate to drain excess oil.