Dissolve salt and dark brown soft sugar in 4 tablespoons warm water. Pour into a large resealable plastic bag with remaining warm water. Place pork in bag, seal and refrigerate overnight.
In a small, microwavable bowl, combine peanut butter, sake, soy sauce, vinegar, honey, groundnut oil, chilli paste, chilli sauce, garlic, ginger, spring onions and coriander. Microwave for 30 seconds to 1 minute to melt peanut butter; mix thoroughly.
Reserve 125g to 175g of this peanut sauce to serve as a sauce for the cooked meat; refrigerate until ready to use. Pour remaining sauce into a resealable plastic bag. Remove pork from brine; discard brine. Rinse pork, pat dry and place in the bag with the peanut sauce marinade. Refrigerate for 8 hours or overnight.
Preheat barbecue for medium heat. Take pork out of peanut sauce marinade; let sit at room temperature for about 20 minutes. Pour marinade in small saucepan. Heat to a boil and boil for 3 minutes. If marinade gets too thick, add a little milk to thin.
Lightly oil barbecue cooking grate and arrange pork on barbecue. When pork reaches 50 degrees C, baste with the boiled marinade. Watch pork carefully after basting so that it doesn't burn. Use a barbecuing spatula to roll the pork around while cooking. Grill pork fillets until internal temperature reaches 68 degrees C; this should take about 15 minutes. Pull off the barbecue and let sit for 5 minutes before slicing into thin rounds.
Warm the reserved sauce in the microwave and place a dollop of the sauce on each plate. Place pork slices on top of sauce to serve.
Sriracha chilli sauce
Can be purchased in Chinese/Oriental speciality shops or online.