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In a heatproof bowl, combine the prunes, vanilla bean and fennel seed bag. In a medium saucepan, bring the wine and Armagnac to a boil. Pour over the prunes; let cool. Cover with plastic and let the prunes macerate for 2 days.
Preheat the oven to 375°. Butter a 10-inch springform pan and lightly dust it with sugar. In a medium saucepan, bring the prunes and their soaking liquid to a boil. Reduce the heat to low and simmer until a syrup forms, about 25 minutes. Strain the prunes over a bowl; remove the fennel seed bag and squeeze the syrup back into the pan. Remove the vanilla bean; scrape the seeds into the syrup and stir. Reserve 12 plump prunes in the syrup. Finely chop the remaining prunes.
In a small saucepan, bring the milk to a simmer. Pour it into a large bowl. Whisk in the flour and 8 egg yolks, 1 at a time. Stir in the chopped prunes.
In a large bowl, beat the egg whites until soft peaks form. Gradually beat in the 1/3 cup of sugar with the salt; beat until the whites are stiff and glossy. Fold one-third of the beaten whites into the prune-egg mixture, then fold in the rest.
Scrape the batter into the pan and bake in the center of the oven for 40 minutes, or until risen and golden brown. Let cool on a rack for at least 30 minutes.
Preheat the oven to 400°. Run a knife around the soufflé and remove the ring. Reheat the soufflé for 15 minutes, or until warmed through. Slice with a serrated knife and transfer to plates. Drizzle with the Armagnac syrup. Serve warm with the whole prunes and sweetened whipped cream.
The soufflé can be prepared through Step 5 earlier in the day.
This airy dessert needs only the grace note of a concentrated wine with similar flavors. Look for a fortified Pinot Gris or a late-harvest Viognier.