Marinate Chicken: In a large bowl, whisk together coconut milk, curry powder, minced garlic, grated ginger, salt, and sugar until combined. Add the chicken thighs, ensuring they are fully coated in the marinade. Cover the bowl and refrigerate for at least 1 hour or up to overnight.

Prepare Coating: On a shallow plate, combine rice flour or cornstarch with a pinch of salt and pepper. Dredge each piece of marinated chicken in the flour mixture, pressing firmly to ensure adherence.

Heat Oil: In a deep skillet or wok, pour enough oil to fill it about 1 inch deep and heat over medium-high until it reaches 350°F (175°C).

Fry Chicken: Add the coated chicken to the hot oil, frying each piece for about 5-7 minutes per side until golden brown and cooked through. Transfer to a wire rack or paper towels to drain excess oil.

Make Spread: While frying, whisk together Thai chili sauce and plain yogurt in a small bowl until smooth. Adjust spice level if necessary.

Assemble Sandwich: Toast sandwich buns until golden. Spread a layer of chili sauce mayo on the bottom, place crispy chicken on top, then add cucumbers, pickled carrots, and fresh herbs.

Serve: Top each sandwich with cilantro and a squeeze of lime if desired. Serve immediately.