Place the chicken breast side down on a large cutting board. Use a pair of very sharp kitchen shears, cut along one side of the backbone, beginning by the thigh and ending up by the neck. Turn the chicken around and cut along the other side of the backbone to completely remove it (you can throw out the backbone, or save it to make stock!). Place the chicken skin-side up on a baking sheet, and press down on it to flatten.
Mix together the butter, miso, garlic, honey, and ginger in a small bowl until a uniform mixture is formed. (I found the miso to be salty enough that I didn't need to add additional salt, but miso pastes can vary, so I recommend tasting the mixture at this point and adding additional salt if you feel it is needed). Rub about half of the miso-butter mixture under the skin of the chicken, and rub the rest evenly over the surface of the skin.
Tent chicken loosely with aluminum foil and bake at 450 degrees for 20 minutes. Remove foil, and bake until a meat thermometer inserted into the breast reads 165 degrees, approximately 20 minutes more.
Let chicken rest for about 10 minutes, then carve and serve, sprinkled with chopped scallions.