Heat a small, dry skillet over medium-high heat. Add the cardamom, coriander, cumin seeds, and the peppercorns, and toast for 30 to 45 seconds, swirling the mixture occasionally until the seeds release their aroma and start to brown. Divide the toasted spice mixture in half. Transfer one half of this mixture to a spice grinder and pulse to a fine powder. (You can prepare the spices up to 1 week in advance and store in an airtight container in a cool, dark place.)
In a blender, combine the remaining toasted spice mixture with the buttermilk, serrano chiles, scallions, 15 of the curry leaves, the garlic, 1 tsp of the cayenne, the ginger, lime juice, and 1 Tbsp of the salt. Pulse until completely smooth and transfer to a large resealable plastic bag. Pat the chicken breasts dry with paper towels. Trim excess fat from the chicken, and cut the flesh into 1 in [2.5 cm] cubes. Add to the marinade. Seal the bag and shake to coat evenly. Refrigerate for 4 hours.
Meanwhile, prepare the dredging mixture. In a large resealable plastic bag, combine the remaining half of the ground spice mixture with the flour, baking powder, baking soda, remaining ½ tsp cayenne, and remaining 1 tsp salt, shaking vigorously to blend. Finely chop 10 of the remaining curry leaves and add them to the dredging mixture. Seal the bag and shake again to mix well.
Once the chicken has marinated, use tongs to lift out half the chicken pieces, shaking off the excess batter, and transfer to the bag with the dredging mixture. Seal the bag and shake to coat evenly. Transfer the chicken pieces to a wire rack. Repeat with the remaining chicken.
In a medium Dutch oven, heat the oil over medium-high heat to 350°F [180°C]. Fry the chicken in batches, turning occasionally, until golden brown and cooked through, 4 to 5 minutes. Using a slotted spoon or a spider, transfer the chicken to paper towels to drain.
After the chicken is cooked, prepare the garnish: Cut the Thai chiles in half lengthwise. In the hot oil left in the pot, deep-fry the chiles and remaining 5 curry leaves until crispy, 30 to 40 seconds. Drain on paper towels.
Put the chicken on a serving plate, garnish with the chiles and fried curry leaves, and serve hot with the maple-vinegar sauce or hot sauce.
© 2018 All rights reserved. Published by Chronicle Books.