To bone the chicken: Carefully remove the wishbone from the breast section of the chicken. Place on a work surface, breast-side down. Slice skin along backbone from neck to tail. Break the legs at the joints. Carefully cut away the meat from the bone. Cut off the wing tips on each side. Continue to cut away the bones from the meat, removing backbone entirely. Cut away the meat from the ribs. Remove all small bones, nerves, and tendons, making sure the skin is kept intact. Feel for the thigh bones; slit the skin of the thigh bones, and scrape and remove each bone. Cover with plastic wrap and refrigerate.
To make the mousse: in a medium skillet, melt butter over medium heat. Add shallot, and cook stirring occasionally, until translucent, about 5 minutes. Add wine, and cook until almost completely evaporated, about 5 minutes. Remove from heat and let stand until cool.
Pulse chicken breast in food processor until smooth. Season with salt and pepper. Add egg whites and egg, and process until combined. With machine running, add cream in three batches, processing until combined after each addition. Add cooled shallot mixture, and process until smooth.
Transfer to a medium bowl, and fold in truffles. Season with salt and pepper. Cover with plastic wrap, and refrigerate for 1 hour.
Cut a piece of heavy-duty aluminum foil approximately 21 inches long. Fold long sides over creating a 1 1/2-inch edge. Lightly butter foil; set aside. Remove chicken from refrigerator and place, skin-side down, on a work surface. Mound the mousse mixture in the center of the chicken. Do not spread it out. Fold the chicken up and over the mousse, slightly overlapping the skin so the mousse is fully enclosed. Turn the chicken, breast-side up, and place in the center of the prepared aluminum foil. Cross the legs, and twist the foil tightly under the legs and at the neck end, so that the foil resembles a gondola. Lightly brush chicken all over with butter. Transfer to refrigerator for 30 minutes.
Preheat oven to 450 degrees. Place chicken in a shallow baking pan, and bake until cooked through, about 1 hour and 15 minutes. Remove from oven and let rest for 15 minutes before slicing.
To make the garnish: In a large skillet, heat butter over medium-high heat. Add shallots, and cook until translucent. Add mushrooms, and cook until mushrooms have released their liquid and it has evaporated, about 5 minutes. Set aside.
With kitchen shears, remove the knuckles from the legs of the chicken. Using a sharp knife, cut under the breast and along the leg joints to remove the breast meat in one piece. Slice the breast lengthwise into 2 pieces. Cutting at a 35-degree angle, slice each breast portion into 5 or 6 pieces. Using a large spatula, reposition the sliced breast onto the chicken. Transfer to a serving platter. Pour Madeira sauce over the chicken, and garnish with cooked mushrooms. Serve immediately.