Preheat the oven to 400°F. If the chicken is already butterflied, move to the next step. If not, cut the backbone out of the chicken with kitchen shears, then turn the bird over and press down on the breastbone until the bird lies somewhat flat.
Cut the jalapenos in half and remove the seeds and stems. Spread the jalapenos on a baking sheet. Roast about 15 minutes, until shriveled. Let the jalapenos cool, then remove the ribs and seeds. Combine the jalapenos, oil, vinegar, paprika, oregano, garlic, and salt in a food processor or blender until it forms a paste.
Transfer the chicken to a glass or non-reactive dish. Pour the marinade over the chicken and flip it a few times so that the marinade coats the chicken well. Cover and refrigerate for 2 to 12 hours.
Preheat the oven to 400°F, and let the chicken come to room temperature while the oven heats up. Remove the chicken from the marinade and place skin side up on a rimmed baking sheet. Roast for about 1 hour until the juices run clear when a sharp knife is inserted into the meaty part of the thigh. Let sit for about 15 minutes on a cutting board, then cut into pieces and serve hot.
Notes
Why spatchcock a chicken?
It cooks more quickly and a bit more evenly, and also allows some nice overall browning. Another somewhat arbitrary reason I discovered is that if you are using all of the racks in your oven, a spatchcocked chicken takes up less headroom. Fun fact.The longer you marinate the chicken, the deeper the flavor. 4 jalapeños may seem like a lot, but once you remove the seeds and ribs, and roast them, you will be left with a soft level nice smoky heat, but hardly a tongue-burning level of spiciness. Either use plastic gloves when handling the jalapeños, or wash your hands right afterwards with plenty of warm water and soap. You only have to touch your eyes or lips once with jalapeño hands to know why.