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Bring sugar and water to a boil in a 1 1/2-quart heavy saucepan over moderate heat, stirring until sugar is dissolved, then wash down crystals from side of pan with a pastry brush dipped in cold water. Boil syrup, without stirring, until thermometer registers 238 to 242°F (soft-ball stage), 3 to 5 minutes.
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While syrup boils, beat egg whites with a pinch of salt in a bowl using an electric mixer at medium speed until foamy. Increase speed to high, then add hot syrup in a slow stream (avoid beaters and side of bowl), beating constantly, and continue to beat until meringue holds stiff glossy peaks and is cooled to room temperature, about 5 minutes. Fold praline into meringue.
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Whisk cream with Cognac in another bowl until it holds soft peaks, then fold into meringue gently but thoroughly. Spoon mixture into ramekins and freeze, loosely covered with plastic wrap, until firm, 2 to 3 hours. Remove foil collars before serving.