VANILLA BUTTERCREAM

Steps:

• Combine whites and salt in a very large bowl. Stir together water and 1 1/3 cups sugar in a 3- to 4-quart heavy saucepan until sugar is dissolved, then bring to a boil over moderate heat, without stirring, brushing any sugar crystals down side of pan with a pastry brush dipped in water.
• When syrup reaches a boil, start beating egg whites with an electric mixer at medium-high speed until frothy, then gradually add remaining 2 tablespoons sugar and beat at medium speed until whites just hold soft peaks. (Do not beat again until sugar syrup is ready.)
• Meanwhile, put thermometer into sugar syrup and continue boiling until syrup registers 238 to 242°F. Immediately remove from heat and, with mixer at high speed, slowly pour hot syrup in a thin stream down side of bowl into whites, beating constantly. Beat, scraping down side of bowl with a rubber spatula, until meringue is cool to the touch, about 10 minutes in a standing mixer or 15 with a handheld. (It is important that meringue is properly cooled before proceeding.)
• With mixer at medium speed, gradually add butter 1 piece at a time, beating well after each addition until incorporated. (Buttercream will look soupy after some butter is added if meringue is still warm. If so, briefly chill bottom of bowl in a large bowl filled with ice water for a few seconds before continuing to beat in remaining butter.) Continue beating until buttercream is smooth. (Mixture may look curdled before all of butter is added but will come back together by the time beating is finished.) Add vanilla and beat 1 minute more.