Combine buttermilk, hot sauce and garlic in a large bowl. Add the chicken and turn to coat well. Cover and refrigerate for 3 hours to 6 hours.
Fill a deep-frying skillet or pot with about 3 inches of oil. Heat over medium heat until the oil reaches 350 degrees F / 180 degrees C.
While the oil is heating up, whisk the flour, cornstarch, garlic powder, onion powder, cayenne powder and salt in a pie plate or large shallow bowl.
One at a time, drain the excess marinade off the pieces of chicken, dredge them in flour mixture, back into the marinade, and then in the flour for a second time. Place the coated chicken on a cooling rack over a baking sheet to rest.
Repeat the process with the remaining chicken pieces, then let them all sit for 10-15 minutes to allow the coating to set.
Cook the chicken thighs for 7-9 minutes or until they turn a light golden color. Flip halfway through cooking. It's okay to fry several pieces of the chicken at a time, but do not overcrowd the pan.
When done, place the chicken pieces on a wire rack over a baking sheet to drain.
While the chicken is cooking, prepare the sauce by adding all the sauce ingredients to a medium saucepan over medium heat. Stir to combine and continue stirring until the butter is fully melted, then remove the pan from the heat before it starts to bubble.
Brush the sauce onto the hot chicken pieces and serve immediately with fries, sliced white bread and pickles.