Heat cast iron skillet over high heat. Cut grapefruit in half horizontally. Sprinkle with sugar. One skillet is hot, place grapefruit in skillet flesh side down. Check occasionally and remove once charred to your liking. There will be smoke! Keep a fan handy or a close window open. Once cool, squeeze juice from grapefruit and set aside.
In a shaker, combine mezcal, 2–4 ounces grapefruit juice, dash of bitters, and a squeeze of fresh lime. Shake vigorously. Strain into a prepared glass of ice with salted rim. Top with club soda. Give 'er a quick stir.
Garnish with a flamed twist.