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1
Combine egg whites and salt in a large bowl, and just let it sit for now.
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2
Stir together brown sugar and water in a small heavy saucepan and bring to a boil over moderately high heat, washing down the sides of the pan with a pastry brush dipped in water. This brings the bubbles down and stops them from boiling over the pan.
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3
When sugar syrup reaches a boil, start beating egg whites with an electric mixer at med-high speed until frothy, then add the lemon juice. Beat at med. speed until whites just hold soft peaks. DO NOT BEAT AGAIN UNTIL SUGAR SYRUP IS READY.
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4
Meanwhile, put candy thermometer into sugar syrup and continue boiling until syrup reaches 238-242u00b0. Immediately remove from heat and pour into a heatproof 1-cup glass measure.
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5
Beating the egg whites at high speed, slowly pour the hot syrup in a thin stream down the side of the bowl into the egg whites.
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6
Beat meringue, scraping down bowl, until meringue is cool to the touch,about 6-7 minute IT IS IMPORTANT THE MERINGUE IS PROPERLY COOLED BEFORE PROCEEDING.
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7
With mixer at medium speed, gradually add butter 1 piece at a time, beating well after each addition until incorporated. (If meringue is too warm and buttercream looks soupy after some of the butter is added, briefly chill bottom of bowl in a large bowl filled with ice water for a few seconds before continuing to beat in remaining butter).
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8
Continue beating until buttercream is smooth. (Mixture may look curdled before all of the butter is added, but it will come back together before beating is finished).
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9
Add vanilla and beat 1 more minute.
