1 - Preheat oven to 400F. Brush four 7 ounce ramekins generously with the butter; coat with granulated sugar. Whisk whites with a mixer until frothy, about 2 minutes. Add cream of tartar; whisk until soft peaks form. Add 1/3 cup granulated sugar; whisk until medium peaks form, about 5 minutes.
2 - Set chocolate in a bowl. Whisk milk into cornstarch and salt in a saucepan. Bring to a simmer. Cook, stirring, until thick, 1 to 2 minutes. Whisk into chocolate (mixture will separate). Whisk in yolks and creme fraiche. Gently fold in egg whites.
3 - Fill ramekins evenly with batter. Bake on a baking sheet, rotating halfway through, until souffles rise but centers are still liquid, about 14 minutes. Dust with confectioners' sugar.
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