Special equipment: a deep-frying thermometer
For the marinade and chicken: In a bowl, whisk together the hoisin, vinegar, soy sauce, sugar and cornstarch. Transfer 1/4 cup of the mixture to a ziptop bag and add the chicken thighs. Marinate, refrigerated, for at least 3 hours, but ideally overnight. Reserve the remaining hoisin mixture to make the General Tso's sauce.
For the crispy chicken: In a large Dutch oven, heat the oil until a deep-frying thermometer registers 350 degrees F.
In a pie dish or shallow bowl, whisk together the buttermilk and eggs. In a separate pie dish or shallow bowl, whisk together the cornstarch and self-rising flour. Mix 3 tablespoons of the buttermilk-egg mixture into the flour mixture; lightly whisk to work the buttermilk into the dredge to create crispy, craggly little bits.
Dunk the chicken first in the buttermilk-egg mixture, then dredge in the flour mixture to coat thoroughly. Gently drop the chicken into the oil, in batches if necessary, and fry until golden brown and the interior registers 160 degrees F, 8 to 10 minutes.
For the General Tso's sauce: Meanwhile, heat the oil in a medium saucepot over medium-low heat. Add the ginger, Szechuan peppercorns and garlic and saute until fragrant, about 1 minute; do not burn. Add the reserved hoisin mixture, bring to a simmer and simmer until thickened, 5 to 8 minutes.
While the fried chicken is still hot, submerge it in the warm sauce to coat.
For the sandwich build: Heat a griddle or skillet over medium heat. Split the rolls in half and butter the insides. Griddle or toast until golden brown.
Place a dunked chicken thigh on each roll and top with a nice sprinkle of toasted sesame seeds. Top with the Chinese Slaw and Chinese Hot Mustard.
In a large bowl, toss together the green onions, carrots, cabbage and chile. In a small bowl, combine the vinegar, soy sauce, sugar and sesame oil. Stir until the sugar dissolves, then pour over the cabbage mixture. Toss until the slaw is coated. Let the flavors meld for at least 30 minutes.
Whisk the boiling water and mustard together in a bowl. Let cool completely.
Mix in the mayonnaise and garlic, then season with salt and pepper if desired.