How to Make It
Preheat the oven to 375°. Cut off the top quarter of the garlic and set it on a sheet of foil. Drizzle the garlic with olive oil, wrap it in the foil and roast for about 1 hour and 20 minutes, until tender.
Meanwhile, in a small saucepan, cover the ancho with water and bring to a boil. Cover and simmer over low heat, turning a few times, until the chile is soft, about 10 minutes; drain. Discard the ancho stems and seeds. Coarsely chop the chile and transfer to a blender.
Squeeze the roasted garlic cloves from the skins into the blender. Add the chicken stock and puree. With the machine on, gradually pour in 1/4 cup of olive oil and puree until incorporated. Pour the sauce into the small saucepan, stir in the soy sauce and season with salt and pepper.
Light a grill. Rub the chicken with olive oil and season with salt and pepper. Grill over moderate heat, turning frequently, until nicely charred and cooked through, about 30 minutes total. Reheat the ancho sauce and serve with the chicken.