First, using a very sharp knife cut away any excess fat or skin from the pork belly slab.
In a bowl, combine sea salt, black pepper, minced garlic, chili paste, and 240mL (1 cup) hoisin sauce. Place the pork belly in a dish rubbing the marinade into the flesh. Refrigerate for 1 hour.
Slice tiger prawns length wise. Rinse and squeeze prawns in a bowl of cold water using 15g salt. Repeat process once more before finally rinsing the prawns again with cold water. Boil the prawns for about 2 minutes in hot water being sure to remove them when cooked through -- do not let the prawns boil too long as this makes the texture rubbery.
When the marinated pork belly is ready, place it in a glass baking pan in the oven at a low broil setting (about 400 degrees) for about 40-50 minutes. You will know that the pork is cooked through when the pork juices run clear when poked. If you're unsure, make a small slice through the middle of a slab of pork belly to check. If you don't have an oven you can always grill the pork belly over charcoal until done.
Once the pork belly is done cooking let it rest for about 30 minutes at room temperature. Afterwards, slice the pork belly into thin (.25cm) slices.
To make the sauce, combine 360mL hoisin, peanut butter, and garlic paste in a small saucepan over medium-low heat. Stir the mixture constantly until the peanut butter is smoothly incorporated into the hoisin. Remove from heat once well-incorporated. Once the sauce has cooled to room temperature use jicama and carrot to garnish.
Prepare hot (but not boiling or scalding) water in a plate for dipping the wrappers.
To assemble your goi cuon, dip a wrapper into the hot water making sure to soak the wrapper evenly (for about 5 seconds). Don't over soak the wrapper as it make it difficult to wrap afterwards. Place the wrapper (now soaked, but should not be softened yet) on a clean plate and wait about 40 seconds for the wrapper to soften. Once the edges are soft and pliable, use a small amount of bean sprouts, cilantro, pork belly, and prawns to make a small 10cm long, 4 cm wide pile towards the bottom edge of your wrapper. To roll this well, take the bottom edge of your wrapper (below your pile) and roll upwards towards the middle of your wrapper once. Make sure that you tightly tuck (without ripping the wrapper) the rolled edge under the pile. Next, fold the left and right outer edges of the wrapper inward. To finish, just roll the pile upwards.
The recipe takes more prep time than it does to really assemble everything. But it's a great sort of throw from the classic goi cuon.
If you freeze the pork belly for about 20 minutes ahead of time, it will make it easier to trim off excess fat and the skin.