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1
In medium heavy saucepan, combine 1 1/2 cups sugar and 6 tablespoons water. Set over low heat and stir until sugar dissolves. Raise heat to high and boil syrup, undisturbed, until candy thermometer registers 240u00b0F, occasionally washing down sugar crystals on side of pan with pastry brush dipped in cold water.
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2
While sugar is boiling, in bowl of electric mixer fitted with whisk attachment, beat egg whites until they just hold soft peaks. Slowly beat in remaining 1/2 cup sugar, then beat until whites just hold stiff peaks.
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3
Decrease speed to medium. Beat in hot syrup in slow, steady stream (try to avoid beaters and side of bowl), then beat until stiff, glossy peaks form and mixture is completely cool, about 8 to 10 minutes (the bottom of the mixing bowl should be cool to the touch).
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4
With mixer on medium, beat in butter, 1 piece at a time, and beat until thickened and smooth. (Mixture will break at first, but, as more butter is beaten in, will thicken and become glossy and smooth.) Beat in vanilla.
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5
Buttercream may be made up to 1 week before serving and chilled in airtight container. If chilled, bring completely to room temperature and beat again before using. Mixture will again appear to break, but will then become glossy and smooth.
