Combine the chili powder, garlic powder, cayenne, cumin, cocoa powder, 1/4 teaspoon salt, 1/2 teaspoon black pepper in a small bowl.
Heat the spice mixture in a small skillet over medium-low just until fragrant, about 1–2 minutes; let it cool to room temperature.
Stir the cooled spices with lime juice, water and cornstarch to make a thin marinade.
Reserve about 2–3 tablespoons of the marinade for the vegetables, then pour the remaining marinade over the chicken breasts and sprinkle the chopped green onion on top. Let the chicken marinate 30 minutes.
Toss the bell pepper strips and red onion strips with the reserved marinade until evenly coated.
Preheat the grill to high heat. Place the chicken directly on the grill and cook about 6 minutes per side, or until cooked through (timing depends on thickness).
While the chicken cooks, place the seasoned peppers and onions on a grill pan and grill until tender and slightly charred, about 8–10 minutes, turning occasionally.
Remove chicken and vegetables from the grill, let chicken rest a few minutes, then serve the chicken breasts topped or surrounded by the grilled peppers and onions.