Whisk together coconut milk, curry powder, minced garlic, grated ginger, salt, and sugar in a medium bowl until fully combined. Add chicken thighs and turn to coat completely. Cover tightly and refrigerate for at least 1 hour, though 4 to 8 hours produces the most flavorful and tender results.

Mix rice flour or cornstarch with salt and pepper on a wide plate. Remove chicken from marinade, allowing excess to drip off. Dredge each thigh thoroughly in the flour mixture, pressing gently so coating adheres evenly. Let rest 5 minutes before frying.

Heat vegetable oil in a deep skillet to 350°F using a thermometer. Oil should be 2 to 3 inches deep. Carefully lower coated chicken into hot oil without overcrowding. Fry for 5 to 7 minutes per side until deep golden brown and internal temperature reaches 165°F. Transfer to wire rack to drain. Keep first batch warm in 200°F oven while frying remaining pieces.

While chicken cooks, combine Thai chili sauce with mayonnaise or yogurt in a small bowl, whisking until smooth. Slice cucumber and prepare pickled carrots. Rinse and dry fresh herbs. Toast buns for 1 to 2 minutes until golden around edges.

Spread chili mayo on both halves of each toasted bun. Place one crispy fried chicken thigh on bottom bun, then top with cucumber slices, pickled carrots, and fresh herbs. Add top bun and press gently.