Preheat the oven to 375°F.
Place the chicken on a large cutting board and turn it over so that it is breast-side-down. Slide your scissors along one side of the spine (inserting at either the top or the bottom of the cavity), and cut along the side of the spine. Repeat this cut along the other side of the spine until you can totally remove the backbone from the chicken. Discard the spine (or save it to make broth).
With the chicken still breast-side-down on the cutting board, use your hands to press the bird as flat as possible, putting your weight over the thighs to press them down. To help the chicken lay more flat, use a sharp knife and slice (from inside the cavity) about ¼ of an inch into the sternum (right between the breasts). Flip the chicken over and prepare to season.
To season the chicken, pull the skin on the breast and legs away from the meat. Using either a spoon or your hands, push the butter under the skin and spread it out as much as possible. Repeat until you've used all the butter. Then, sprinkle with the salt and pepper.
Place the chicken on a rimmed baking sheet or in a roasting pan (or large heavy-bottomed pot). Bake at 375°F uncovered for 50 minutes. Be careful to watch that the skin doesn't burn.
Once the chicken is finished baking, set it aside to rest for 5 minutes. Then serve and enjoy!