In a mixing bowl, whisk together the buttermilk, fish sauce, garlic powder, white pepper, paprika, and sugar until smooth. Submerge the chicken thighs or breasts in this marinade, cover, and refrigerate for at least one hour, preferably overnight.

In a small bowl, combine the rice vinegar, sugar, and salt, stirring until sugar is dissolved. Toss in cucumber slices and allow to soak for at least 30 minutes at room temperature.

In a large mixing bowl, combine shredded cabbage and carrots. Drizzle with rice vinegar, sprinkle the sugar, and add a pinch of salt. Toss well and let sit for 10 minutes.

In a shallow dish, mix all-purpose flour, baking powder, and salt. Remove chicken from the fridge, coat each piece generously in the flour mixture, pressing gently to adhere.

Heat vegetable oil in a deep skillet over medium heat until 350°F (175°C). Carefully lower chicken into the hot oil and fry for 6-8 minutes per side until golden brown and cooked through. Check internal temperature reaches 165°F (74°C).

In a small bowl, combine mayonnaise, Sriracha, sweet chili sauce, and lime juice. Stir until smooth, adjusting Sriracha to taste.

To assemble, spread spicy mayo on the bottom half of a toasted bun, place the crispy fried chicken, add slaw and pickled cucumbers, and top with the other half of the bun.