How to Make It
Make the soufflés In a medium saucepan, bring the milk to a boil. In a large bowl, using a hand mixer, beat the egg yolks with 6 tablespoons of the sugar until light and fluffy, about 5 minutes. Beat in the flour. Gradually beat in the hot milk. Return this pastry cream to the pan and cook over moderately low heat, whisking often, until thickened and bubbles appear, about 2 minutes. Remove the pan from the heat. Press a piece of plastic directly on the pastry cream and keep warm.
In a small saucepan, moisten the 1 1/2 cups of sugar with 1/4 cup of water and bring to a simmer over moderate heat, stirring, until the sugar dissolves, about 3 minutes. Cook, brushing down the sides of the pan with a wet pastry brush and swirling the pan occasionally, until the syrup turns deep amber brown and just begins to smoke, about 8 minutes. Immediately whisk the hot caramel into the pastry cream. Transfer the caramel cream to a large bowl and press a piece of plastic directly on the cream. Refrigerate until cool, at least 3 hours.
Meanwhile, prepare the Citrus Whipped Cream In a large bowl, using a hand mixer, beat the cream with the sugar, vanilla seeds and grapefruit and orange zests until it holds a soft peak. Refrigerate until ready to use.
Preheat the oven to 400°. Brush six 9-ounce soufflé dishes with butter and sprinkle with sugar, shaking out the excess. Refrigerate until the butter is firm. Transfer the soufflé dishes to a rimmed baking sheet. Whisk the caramel cream to loosen.
In a large bowl, using a hand mixer, whip the egg whites at moderately low speed until they hold a soft peak. Gradually beat in the remaining 6 tablespoons of sugar until the whites hold a firm glossy peak. Fold the whites into the caramel cream until no streaks remain. Scrape the mixture into the prepared soufflé dishes. For a nice flat top on the finished soufflés, use a large offset spatula or the back of a chef’s knife to scrape the edge across the tops of the dishes to level.
Transfer the soufflé dishes to a rimmed baking sheet and bake for about 20 minutes, until the soufflés are puffed but still moist in the center. Serve at once with the Citrus Whipped Cream.