Combine buttermilk, salt, Tabasco sauce, and garlic powder in a large ziptop bag. Add turkey. If you can't get the bag sealed, set it inside a bowl in case it spills or leaks. Refrigerate for 8 to 12 hours.
Discard marinade and pat turkey dry with paper towels. Set on a roasting rack set inside a roasting pan.
Pour melted butter all over turkey and season generously with salt and pepper.
Pour chicken broth into bottom of pan and add celery, carrots, onion, and parsley
Preheat oven to 350 degrees.
Roast in oven for about 1 1/2 hours or until internal temperature measure 165 degrees. Let rest 20 minutes before slicing.
To make gravy, Strain drippings into a 2 cup measuring cup. Add chicken broth if needed to make 2 cups. Add drippings to a medium saucepan. Add flour and whisk in. Place over medium-high heat and bring to a simmer, whisking continuously. Cook and whisk another minute or two or until thickened,