Directions:

• Cut each chicken thigh lengthwise into 3 equal pieces. Stir together vinegar, 3 tablespoons of the ginger-garlic paste, and 1 tablespoon of the kosher salt in a large bowl. Add chicken, and stir to coat. Marinate chicken, covered, in refrigerator for 45 minutes.
• Process mint, onion, water, chiles, ginger, and 3 cups of the cilantro in a blender until almost smooth, about 1 minute, adding additional water, 1 tablespoonful at a time, as needed to loosen mixture, and stopping occasionally to scrape down sides of bowl as needed. Add remaining 3 cups cilantro; process until smooth, about 1 minute. Transfer chutney to a medium bowl. Stir in lemon juice, sugar, and kosher salt. Mint Chutney may be stored in an airtight container in refrigerator up to 2 days.
• Stir together 1 1/4 cups yogurt, 1/4 cup chili powder, garam masala, turmeric, 2 tablespoons of the kosher salt, and remaining 3 tablespoons ginger-garlic paste in a separate large bowl; whisk until combined. Remove chicken from marinade, shaking off excess liquid, and transfer to yogurt mixture, coating pieces evenly. Cover and marinate chicken in refrigerator for 3 to 4 hours.
• Stir together amchur, hot chili powder, and sea salt in a small bowl. Spread onion slices evenly on a large baking sheet, and sprinkle evenly with amchur mixture. Cover and refrigerate until ready to use. Amchur Onions can be stored in refrigerator up to 2 days.
• Stir together yogurt and scallions in a medium bowl until well combined. Refrigerate until ready to use. Scallion Yogurt can be stored in refrigerator up to 2 days.
• Whisk together flour, cornstarch, pepper, and remaining 2 tablespoons kosher salt in a 9- x 13-inch dish or other large shallow dish. Remove chicken pieces, 1 at a time, from marinade, shaking off excess. Place in flour mixture, turning to coat both sides of chicken; transfer to a baking sheet.
• Pour peanut oil into a large pot to a depth of 2 inches, and heat over medium to 325°F. Working in 3 batches, add chicken to oil, and fry until lightly browned, about 6 minutes per batch. Remove chicken from oil, and transfer to a baking sheet lined with a wire rack.
• Increase heat under oil to medium-high; heat oil to 400°F. Working in 3 batches, fry chicken a second time until deeply brown and crispy, 2 to 4 minutes per batch. Transfer to wire rack.
• Melt butter in a medium saucepan over medium. Add chaat masala and Kashmiri chili powder, and stir until combined. Dip each piece of chicken into butter mixture, making sure chicken is coated in spices; allow excess butter to drip off. Arrange chicken pieces evenly on bottom bun halves. Top evenly with Mint Chutney, Scallion Yogurt, and Amchur Onions; replace top bun halves, and serve.