To prepare souffles: Completely coat the interiors of four 8-ounce ramekins (3 1/2-inch diameter) with 1 teaspoon butter. Refrigerate until the butter is cold and set, about 2 minutes. Coat again with the remaining 1 teaspoon butter. Over a small bowl, add 1 teaspoon sugar to each ramekin and turn and shake to completely coat. Shake out any excess sugar into the bowl. Refrigerate the ramekins for at least 15 minutes and up to 1 day.

Position rack in center of oven; preheat to 375 degrees F.

Place bananas on a baking sheet lined with foil. Roast until the peels are completely blackened and the flesh is soft, 20 to 30 minutes. Remove the bananas and return the baking sheet to the oven.

When the bananas cool enough to handle, peel and transfer to a blender. Add 2/3 cup cream and milk and puree until smooth. Transfer the mixture to a heavy medium saucepan. Scrape the seeds from the vanilla bean into it and bring to a simmer over medium heat.

Whisk egg yolks and 2 tablespoons sugar in a medium bowl until light and fluffy, about 1 minute. Whisk in flour, then slowly whisk in the hot banana mixture. Return the mixture to the saucepan and cook, stirring constantly, over medium-high heat until the custard is very thick and just beginning to bubble, about 2 minutes. Transfer to a large bowl. Press a sheet of plastic wrap directly on the surface of the custard to prevent a skin from forming and let cool to room temperature, about 30 minutes.

Meanwhile, prepare caramel sauce: Heat 3/4 cup sugar in a heavy medium saucepan over medium heat. Cook, tilting and swirling the pan to distribute the caramelizing evenly but without stirring, until the sugar melts and turns a deep amber color, about 5 minutes. As soon as it's deep amber, remove from heat and whisk in cream and butter until completely smooth. Stir in fleur de sel, cover and set aside.

Whisk egg whites on medium-high speed in the bowl of a stand mixer fitted with the whisk attachment until foamy, about 1 minute. Slowly sprinkle in the remaining 4 tablespoons granulated sugar and beat until peaks are nearly stiff, about 3 minutes. Whisk one-third of the whites into the cooled custard. Fold in another one-third of the whites, then fold in the remaining whites. Divide the mixture evenly among the prepared ramekins.

Bake the souffles on the sheet pan in the oven until they have risen but are still a bit liquid in the center, 15 to 20 minutes. Dust with confectioners' sugar and drizzle each souffle with 1 teaspoon caramel sauce. Serve immediately.