In a large bowl, combine coconut milk, curry powder, minced garlic, minced ginger, salt, and sugar to create a flavorful marinade. Add the chicken thighs, ensuring they are fully coated. Cover and refrigerate for at least 1 hour, or ideally overnight.

On a large plate, mix rice flour (or cornstarch) with salt and pepper. Once the chicken has marinated, remove it from the fridge, letting excess marinade drip off. Dredge each chicken thigh in the flour mixture, pressing down to ensure even coating.

Heat vegetable oil in a deep skillet or fryer to 350°F (175°C). Carefully place the coated chicken thighs into the hot oil, avoiding overcrowding. Fry for 5-7 minutes per side until golden brown and cooked through.

In a small bowl, whisk together Thai chili sauce and mayonnaise (or yogurt) until smooth and creamy. Adjust spice level as desired.

Toast the buns until golden brown. Spread a generous layer of the spicy mayo on both halves of the buns. Layer the fried chicken on the bottom bun, adding cucumbers, pickled carrots, and Thai basil or cilantro. Press the top bun gently down to complete the sandwich.