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1
Preheat the oven to 400F (205C) and position a rack in the lower third of the oven. Lightly butter the bottom and sides of a 2-quart (2-L) souffle dish. Dust the souffle dish with sugar (so that it adheres to the butter) and knock out the excess.
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2
In a large saucepan with high sides, combine 1 cup (200 g) sugar, 1/4 cup (60 ml) water, and the corn syrup. Stir the mixture gently so you don't splash any of it up the sides of the pan. Turn the heat to medium-high and continue stirring until the sugar dissolves. Increase the heat to high, stop stirring, clip on a candy thermometer (making sure the bulb is immersed in the sugar but not touching the pan), and allow the mixture to boil. Once it begins to turn a rich, dark caramel color and the thermometer reads 345F (175C), 5 to 8 minutes (don't worry if it takes longer, the actual time is reliant on so many factors), remove it from the heat; do not overcook. Gently and slowly stream in the heavy cream (it will bubble up, so be careful). Stir in the fleur de sel. Return the mixture to medium-low heat; don't worry if the caramel mixture begins to harden-it will easily melt again as it reheats. Add the milk and stir to combine. Reduce the heat to low. Leave the mixture on the heat while you prep the egg yolks.
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3
In a large bowl, whisk together the egg yolks and 1/4 cup (50 g) sugar. Sprinkle the mixture with the flour, then the cornstarch, and whisk until completely combined. Pour one-third of the caramel mixture into the egg mixture, whisking the egg mixture constantly. Slowly stream in the rest of the caramel while whisking constantly until combined. Set the bowl aside.
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4
In the bowl of a standing mixer fitted with the whisk attachment (or using a whisk and bowl and a ready arm), whisk the egg whites on high speed for 1 minute. Sprinkle the cream of tartar and salt over the whites and continue whisking on high speed until the egg whites form soft peaks. Slowly stream in the remaining 1/4 cup (50 g) sugar, and continue beating until stiff (but not dry) peaks form. Using a rubber spatula, gently fold one-quarter of the stiff egg-white mixture into the caramel mixture until almost combined. The caramel mixture will begin to lighten. Fold another quarter of the egg-white mixture into the caramel mixture until nearly combined. Finally, add the remaining egg-white mixture to the caramel mixture and fold gently until completely combined.
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5
Transfer the souffle batter to the prepared dish. For an even rise, run your thumb around the inside edge of the dish to wipe away any stray batter. Place the souffle in the oven, and immediately reduce the oven temperature to 375F (190C). Avoid opening the oven door during the recommended baking time. Bake until the souffle is puffy and dry to the touch, and the center is just about set but slightly jiggly (that is, slightly jiggly, not crazy ripply), 22 to 30 minutes.
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6
Transfer the hot souffle dish to a serving platter and serve immediately as is or with unsweetened whipped cream, if you like.
