Use a 3-4 pound chicken. Turn it breast side down on a cutting board and starting at the bottom, cut upward with kitchen shears, along the backbone. Cut along the other side and remove backbone. Turn the chicken and flatten it in true butterfly style by applying pressure with your palm. (Here's a video of a more complete method and a more refined method, which makes it easier to cut for serving.)
Spatchcocked Chicken with Tomatoes
Preheat oven to 500 degrees.
Season spatchcocked chicken with coarse salt and ground pepper and place, breast side up, in a pan (or on a cookie sheet) with 3 unpeeled garlic cloves, smashed. With the tip of a paring knife, pierce 1 pint cherry tomatoes. Add to pan and drizzle tomatoes with 1 teaspoon olive oil; season with salt and pepper. Pour 1/2 cup dry white wine and 1/3 cup water into pan. (we forgot the wine and it was delicious...) Roast chicken until juices run clear when pierced between breast and leg (to 165 degrees at the thickest part of the meat) 30-40 minutes. Let rest 5 minutes. Sprinkle with 1/4 cup basil leaves before carving.
Taken from Martha Stewart Everyday Cooking July, 2010.
Also see Martha Stewart's website recipe.