Prepare the Quick Pickles (Ahead of Time): Thinly slice one small cucumber and two shallots. In a small bowl, combine ½ cup rice vinegar, 2 tablespoons sugar, and ¼ teaspoon salt. Stir until the sugar and salt dissolve. Add the sliced cucumbers and shallots to the vinegar mixture and stir to coat. Cover and refrigerate until ready to assemble the sandwiches.

Marinate the Chicken: Pat 1.5 lbs boneless, skinless chicken thighs very dry with paper towels and cut them into sandwich-sized pieces. In a medium bowl, mix 2 tablespoons fish sauce, 1 tablespoon soy sauce, 1 tablespoon sugar, 2 cloves minced garlic, ½ teaspoon white pepper, and 1 teaspoon cilantro root (if using). Add the chicken to the marinade, ensuring each piece is well coated. Cover and refrigerate for at least 30 minutes, or ideally 2-4 hours for maximum flavor.

Make the Creamy Thai Chili Mayo: In a small bowl, mix ½ cup mayonnaise, 2 tablespoons sriracha (or 3 tablespoons sweet chili sauce), 1 tablespoon fresh lime juice, ½ teaspoon fish sauce, ¼ teaspoon sugar, and 2 tablespoons finely chopped fresh cilantro. Stir until well combined and smooth. Taste and adjust seasoning if needed, then refrigerate until ready to use.

Prepare the Breading Station: In a shallow dish or pie plate, combine 1 cup all-purpose flour, ½ cup cornstarch (or rice flour), 1 teaspoon baking powder (optional), 1 teaspoon garlic powder, 1 teaspoon onion powder, ½ teaspoon white pepper, and ½ teaspoon salt. Whisk well to combine. Remove the marinated chicken from the refrigerator.

Fry the Chicken: Pour enough neutral oil into a large pot to reach about 2-3 inches deep. Heat the oil over medium-high heat until it reaches 350°F (175°C). Dredge a marinated chicken thigh in the flour mixture, pressing firmly to ensure it's fully coated. Carefully lower 2-3 pieces of chicken into the hot oil, making sure not to overcrowd the pot. Fry for 4-6 minutes per side or until golden brown and cooked through (internal temperature of 165°F). Once cooked, remove the chicken from the oil and place on a wire rack over a baking sheet to drain excess oil. Season immediately with a pinch of salt. Repeat with remaining chicken.

Assemble the Sandwiches: Lightly toast your brioche buns if desired. Spread a generous layer of the Creamy Thai Chili Mayo on both halves of each bun. Place one or two pieces of hot, crispy fried chicken on the bottom bun, then top with a handful of quick-pickled cucumbers and shallots. Garnish with fresh cilantro and mint leaves. For some extra heat, add a few slices of fresh chili. Crown with the top bun and serve immediately!