Combine the chicken thighs, buttermilk, pickle juice, garlic cloves, peppercorns, bay leaves, salt, and garlic powder in a large ziplock bag. Shake together, seal, and refrigerate for at least 8 hours and up to 24 hours.
Remove the chicken from the brine and pat dry. Fill a deep heavy pot with 2 inches of peanut-oil and heat to 350°F over medium-high.
Combine the flour, salt, garlic powder, smoked paprika, pepper, fennel, and cayenne in a medium bowl. When the oil has reached the target temperature, take a piece of chicken and dredge in the spiced flour mixture until fully covered.
Carefully place in the heated oil. Cook until the chicken reaches an internal temperature of 165°F, about 6 to 10 minutes. Transfer to a plate covered with paper towels. Repeat with the remaining chicken.