Whisk the buttermilk with 1 teaspoon salt and 1 teaspoon black pepper, add the chicken and refrigerate for 4 to 12 hours.
Stir the flour, cornstarch, garlic powder, remaining salt and remaining black pepper in a wide bowl, then drizzle in 3 tablespoons of the buttermilk marinade and rub it into shaggy clumps.
Lift each thigh from the marinade, press it firmly into the flour mixture and let the coated pieces stand on a rack for 10 minutes.
Heat the canola oil in a deep heavy pot to 350°F, leaving at least 3 inches of headroom.
Fry 2 thighs at a time for 6 to 8 minutes, turning once and adjusting the heat to keep the oil between 335°F and 350°F, until the crust is deep golden and the chicken reaches 165°F.
Drain the chicken on a wire rack set over a sheet pan, not on paper towels, and rest it for 5 minutes.
Toast the buns cut-side down in a dry skillet for 1 to 2 minutes, then spread both halves with mayonnaise and stack each with 1 fried thigh and 4 pickle chips.