Dissolve the salt and brown sugar in 1/4 cup warm water. Pour into a large resealable plastic bag with remaining 2 3/4 cups water. Place pork in bag, seal, and refrigerate overnight.
In a small, microwavable bowl, combine peanut butter, sake, soy sauce, vinegar, honey, peanut oil, chili paste, chili sauce, garlic, ginger root, green onions, and cilantro. Microwave for 30 seconds to 1 minute to melt peanut butter; mix thoroughly.
Reserve 1/2 to 3/4 cup 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 grill 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 grill grate, and arrange tenderloins on grill. When pork reaches 125 degrees F (50 degrees C), baste with the boiled marinade. Watch pork carefully after basting so that it doesn't burn. Use a bbq spatula to roll the pork around while cooking. Grill pork tenderloins until internal temperature reaches 145 degrees F (63 degrees C); this should take about 15 minutes. Pull off the grill, 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.