Put an oven rack in the middle position and preheat the oven to 350 degrees F. Butter a 6-cup souffle dish or casserole.
Melt the chocolate and butter in a large bowl set over a pan of barely simmering water, stirring frequently. (Or in a microwave-safe glass bowl in a microwave at 50 percent power for 2 to 3 minutes, stirring well every 30 seconds until the chocolate is melted and smooth.) Cool completely.
Scrape the seeds from the vanilla bean into the cooled mixture and add salt and 6 tablespoons sugar. Add yolks 1 at a time, whisking well after each addition. Whisk in the flour.
Beat the whites with a pinch of salt in a bowl using an electric mixer at high speed until they hold soft peaks. Add the remaining 6 tablespoons sugar, a little at a time, beating until the whites hold stiff glossy peaks.
Whisk about 1/4 of the whites into the chocolate mixture to lighten the batter, then fold in the remaining whites gently but thoroughly. Pour the batter into the souffle dish, spreading it evenly.
Bake about 30 to 35 minutes or until the center is still jiggling and a toothpick inserted in the center comes out with moist crumbs adhering to it. Serve the souffle warm.
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