Gently separate the skin of the chicken from the meat with your fingers.
Whisk together the soy, sambal, mustard, pasilla/ancho powder, cumin, salt, lime zest and juice and a few cracks of black pepper. Pour into a ziptop bag, add the chicken and marinate, refrigerated, overnight.
When ready to cook, place a cast-iron skillet on the bottom rack of the oven and preheat the oven to 500 degrees F.
Remove the chicken from the marinade and pat dry.
Carefully remove the hot skillet and lower the oven temperature to 450 degrees F. Add the vegetable oil to the skillet, place the chicken breast-side down in it and return it immediately to the oven. Roast until well browned, 20 to 30 minutes. Gently flip the chicken and cook until the breast registers 160 degrees F, about another 10 minutes. Let rest for 10 minutes on a cutting board. Slice and serve with Creamy Cilantro Sauce.
Put the cilantro, oregano, honey, mustard, cumin, serrano, lime zest and juice and garlic in a food processor. Process until smooth, scraping down the sides to evenly distribute. Add the mayo and pulse until combined. Season with salt and pepper.