Preheat the oven to 300 degrees Fahrenheit. In a small bowl, mix together the cashews and Sriracha sauce until the nuts are evenly coated. Line a small baking pan with foil, spread the coated cashews on it, and bake in the preheated oven for about 20 minutes until the nuts are dry. Once done, carefully remove the nuts from the foil, let them cool, chop them roughly, and set them aside in a small bowl.
In a large bowl, whisk together the sesame oil, ½ cup soy sauce, hoisin sauce, black pepper, and hot sauce (if using). Add the chicken thighs to the bowl and stir until they are coated with the mixture. Refrigerate the chicken thighs until you're ready to cook them. To prepare the basting sauce, combine the remaining 3 tablespoons of soy sauce, brown sugar, and ginger in a small bowl. Whisk the ingredients together until the sugar has completely dissolved.
To begin cooking the chicken, prepare a charcoal grill by lighting a fire, ensuring that about one-third of the cooking area remains free of coals. Wait until the coals are covered with gray ash and the temperature reaches a medium level (you should be able to hold your hand about 5 inches above the coals for 5 to 7 seconds) before proceeding. (For a gas grill, turn one burner to high, leaving the others off, then close the lid and allow it to heat for 15 minutes.)
Using tongs, take the chicken thighs out of the marinade and place them directly over the coals on the grill. Turn them every few minutes, cooking until they are well-browned but not overly crusty, typically around 8 to 10 minutes. Next, use a pastry brush to begin basting the thighs with the soy, ginger, and sugar mixture. Continue cooking for an additional 8 to 10 minutes, basting and turning the chicken regularly until it develops a glossy lacquer. If the chicken starts to char too quickly, move it to the part of the grill without coals.
Once cooked, transfer the chicken to a warmed platter. Sprinkle the chopped cashews, sliced scallions, and chopped cilantro over the chicken, and then it's ready to serve.