Mix together the five-spice powder and sugar in a small bowl, and then add salt and pepper to taste.
For the pork belly:
Preheat the oven to 350 degrees F. Heat the oil in a cast-iron skillet over high heat. Rub the dry rub over the whole piece of pork belly, making sure to push the rub into the meat. Once the pork belly is covered, sear it in the skillet until browned, 4 minutes per side, to ensure that you have sealed in the juices. Pull the pork belly out of the skillet and let it rest. Without emptying the meat's juices from the skillet, saute the garlic and onions over medium heat until browned, 2 minutes. Add the soy sauces, sugar, vinegar, star anise and 1/2 cup water, and then add the pork belly, making sure it is no more than halfway covered by the liquid. Place the skillet into the oven and let it braise until it is very moist and tender, and can be pulled apart with ease, about 3 hours 30 minutes. Let the pork belly rest, and then chop it into smaller pieces.
For the salad:
Meanwhile, shred the carrots and daikon into thin strips using a Chinese mandolin. Mix together the vegetables with the sugar, vinegar, chili garlic and fish sauce. Let the mixture sit and rest.
For the mayonnaise:
Whisk together the mayonnaise, Sriracha, lime juice and rice vinegar in a small bowl and pour into a small squeeze bottle or set aside.
Heat the tortillas on a griddle over high heat. Add a small serving of the pork belly to each one, and top with the daikon and carrot salad, Sriracha mayonnaise and chopped cilantro, and serve.