Combine buttermilk, paprika, and cayenne pepper in a large bowl. Add chicken thighs, and turn to coat. Cover and marinate in refrigerator at least 6 hours or overnight. Place flour in a shallow dish. Lightly whisk together eggs and water in a second shallow dish. Place panko in a third shallow dish. Remove chicken from marinade; discard marinade. Sprinkle chicken with salt. Dredge in flour, shaking off excess. Dip in egg mixture, allowing excess to drip off. Dredge in panko, pressing to adhere. Coat chicken on both sides with cooking spray. Lightly spray air fryer basket with cooking spray. Place chicken in single layer in the basket, and cook in batches at 400°F until a thermometer inserted in chicken registers 165°F and coating is golden brown and crispy, 16 minutes, turning chicken over halfway through cooking. To serve, place a chicken thigh on each plate, and drizzle with 1/2 teaspoon hot sauce. Serve immediately.