Place oven rack in upper-middle position and preheat oven to 500°F (260°C). Using sharp kitchen shears, remove the spine from the chicken and cut the spine into 5 to 6 one-inch-long pieces. Set spine aside. Flatten chicken by placing skin side up on a cutting board and applying firm pressure to the breast bone.
Transfer to a wire rack set in a foil-lined rimmed baking sheet. Position the chicken so that the breasts are aligned with the center of the baking sheet and the legs are close to the edge.
Drizzle chicken with 1 tablespoon of oil. Combine 1 tablespoon sea salt, 1/2 teaspoon ground black pepper, and 1 teaspoon baking powder in a small bowl, mix with a fork, and sprinkle all over the chicken. Sprinkle chicken with herbs, if using, and rub chicken to distribute seasoning evenly all over the skin.
Roast chicken until the thickest part of the breast close to the bone registers 150°F on an instant-read thermometer and the joint between thighs and body registers at least 155°F, about 45 minutes. Reduce heat to 450°F if chicken starts to darken too quickly.
Meanwhile, heat the remaining 1 tablespoon oil in a small saucepan over high heat until shimmering. Add chicken spine and cook, stirring frequently, until well browned about 3 minutes. Add onion, carrot, and celery and cook, stirring frequently, until beginning to brown, about 3 minutes. Add bay leaf and deglaze with vermouth or sherry and 1 cup water, using a wooden spoon to scrape up any browned bits from the bottom of the pan.
Reduce heat to maintain a simmer and cook for 20 minutes. Strain out solids and return liquid to pan. Boil over medium-high heat until approximately 1/3 cup remains, about 7 minutes. Whisk in soy sauce, butter, and lemon juice off the heat. Season to taste with salt and pepper.
Remove chicken from oven, transfer to cutting board, tent loosely with foil, and allow to rest 5 minutes before carving. Serve with hot jus and Luxurious Potato Puree.