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In a 3-quart saucepan, combine 1 cup of the sugar with the lemon juice and whisk to combine; the sugar will resemble moist sand. Slowly caramelize the sugar by heating it for 8 1/2 to 9 minutes over moderate heat, stirring constantly with a whisk to break up any lumps until it is a deep chestnut color. The sugar will become clear as it liquefies, then it will brown as it caramelizes. Remove the saucepan from the heat and carefully (to avoid splattering) add the cream. Whisk vigorously to combine. Transfer the "burnt" caramel sauce to a 1-quart bowl.
Heat 1 inch of water in the bottom of a double boiler over moderate heat. In the top of the double boiler, combine the egg yolks with the remaining 1/2 cup sugar and the hazelnut liqueur. Whisk vigorously over the heat until soft peaks form and the sabayon becomes light and airy, about 8 minutes. Remove from the heat and gradually whisk in the "burnt" caramel sauce until thoroughly blended. Serve immediately.