Step 1Combine the jalapeños, cilantro, garlic, mayonnaise, sour cream, chile paste, lime juice and vinegar in the jar of a blender and blend on high speed until smooth. With the blender running, slowly drizzle in the olive oil. Season to taste with one-half teaspoon salt and one-eighth teaspoon pepper, or to taste. This makes about 1 cup sauce, which can be stored in a sealed container in the refrigerator for up to 1 week.
Step 1Adjust an oven rack to the upper-middle position and heat the oven to 450 degrees.
Step 2In a small bowl, combine the cumin, paprika, garlic, vinegar, salt, pepper and oil and massage together with your fingertips.
Step 3 Dry the chicken thoroughly. Separate the chicken skin from the breasts. Spread the spice mixture evenly over the chicken and all under the skin.
Step 4Set a wire rack on a rimmed baking sheet lined with aluminum foil. Position the chicken so that the breasts are in the center of the baking sheet and the legs are close to the edges. Roast until the thickest part of the breast close to the bone registers 160 degrees on an instant-read thermometer, 35 to 45 minutes.
Step 5Transfer the chicken to a cutting board, tent loosely with foil, and rest for 10 minutes, then carve and serve with the spicy jalapeno sauce, if desired.