For the Batter: In a large bowl, combine flour, powdered sugar, salt, baking soda, baking powder, and scrapings of one large vanilla bean, mixing by hand until the seeds are well dispersed.
Add buttermilk, melted butter, egg whites, and vanilla extract and whisk until smooth.
For the Waffles: Set waffle iron to medium and preheat until indicator is ready. For an 8-inch square machine, scrape in roughly half the batter. For a 7-inch Belgian machine, scrape in about one-third. Close lid and cook until waffle is golden brown but still steaming, 4 1/2 to 6 minutes depending on the depth and heat of your machine. Serve immediately.