Whisk buttermilk and salt in a large bowl. Toss chicken in the mixture. Cover tightly, refrigerate, and marinate for a minimum of 2 hours, or up to 24 hours.
After marinating, remove chicken from buttermilk, wipe off excess. Keep thin buttermilk coating. Refrigerate until use. Line baking sheet with foil, add cooling rack on top. Set aside.
Heat oil in Dutch oven or heavy-bottomed pot to 350°F. If it exceeds 375°F, lower heat to avoid burning. Without thermometer, test after 10 mins with a chicken piece. If it bubbles, it's ready; if not, wait.
As oil heats, mix flour, panko, pepper, cayenne in a bowl. Coat chicken with the mixture evenly.
Fry chicken bites in batches. Shake off excess flour, carefully lower into hot oil using a kitchen spider. Fry until golden brown and cooked (about 5 mins). Drain on prepared rack. Repeat.
As chicken cools, make sauce. In a saucepan, mix vinegar, water, sugar, sambal, garlic, ginger, soy sauce. Bring to low boil, dissolve sugar. Add cornstarch slurry, stir till thickened. Cool 5 mins.
Coat chicken bites with sauce in a bowl. Shake off excess and place on serving plate. Optionally, garnish with crushed peanuts and cilantro.