Marinate the chicken thighs in buttermilk for at least 2 hours, preferably overnight.
In a bowl, mix flour, garlic powder, paprika, cayenne, salt, and pepper.
Remove the chicken from the buttermilk and allow excess to drip off, then dredge in the flour mixture, pressing it in to adhere.
Heat vegetable oil in a deep skillet or fryer to 350°F (175°C).
Fry the chicken in batches for 5-7 minutes until golden and cooked through.
While the chicken is frying, combine melted butter and Buffalo sauce in a bowl.
Once the chicken is cooked, transfer it to a paper towel-lined plate to drain excess oil.
Toss the fried chicken in the Buffalo sauce mixture until well coated.
Toast the burger buns lightly on the skillet or under the broiler until golden.
Assemble the sandwich with crispy chicken, lettuce, tomato, and pickles on the toasted bun.