In a large mixing bowl, whisk together the buttermilk, hot sauce, kosher salt, and black pepper. Add the chicken thighs and turn to coat, then cover and refrigerate for at least 20 minutes or up to overnight.

In a shallow dish, stir together the all-purpose flour, cornstarch, sweet paprika, garlic powder, onion powder, 1/2 teaspoon cayenne pepper, and 1/2 teaspoon fine salt until evenly combined.

Remove each chicken thigh from the marinade and allow the excess liquid to drip off. Dredge every piece well in the seasoned flour, pressing gently so the coating adheres, and place on a wire rack to rest for 10 minutes.

Pour the vegetable oil into a heavy skillet or Dutch oven to a depth of about 1 inch. Heat over medium-high heat until a deep-fry thermometer registers 350°F.

Working in batches, fry the chicken thighs for 5 to 7 minutes per side, turning occasionally, until deep golden and crunchy. The thickest part should read 165°F on an instant-read thermometer; transfer to a wire rack to drain.

In a heatproof bowl, combine 1/2 cup of the hot frying oil with the 2 tablespoons cayenne pepper, brown sugar, chili powder, smoked paprika, 1/2 teaspoon garlic powder, and 1/2 teaspoon salt until smooth. Brush the spicy oil generously over both sides of the fried chicken.

If desired, spread the cut sides of the brioche buns with the unsalted butter and toast them in a clean skillet until golden brown.

Place a glazed chicken thigh on each bun bottom, then top with dill pickle chips and a generous scoop of coleslaw. Close with the top bun and serve immediately.