Combine the brown sugar, tamari, ginger, crushed garlic, 12 tbsp togarashi, 1 tbsp togarashi, and 1 cup water in a saucepan and bring to a boil.
Reduce heat to low and simmer until the sauce has reduced by half, stirring occasionally.
Strain the reduced sauce through a fine mesh strainer into a bowl and allow it to cool for later use.
While the sauce cools, cut the chicken thighs into 1-inch cubes and season with salt and pepper.
Thread about 3–4 pieces (2–3 oz) of chicken onto each skewer.
Sprinkle the skewers with togarashi; this can be done up to 24 hours in advance.
Preheat a grill or grill pan over medium-high heat and grill the skewers, turning as needed, until the chicken is cooked through.
Warm the reserved sauce in a sauté pan over medium heat while the chicken cooks.
Toss the cooked skewers in the warmed sauce until well coated.
Stack the coated skewers on a plate in a crisscross pattern, drizzle with extra sauce, and garnish with the sliced, chilled scallions.