Combine egg whites and salt in bowl of a standing electric mixer or other large bowl. Stir together sugar and water in a small heavy saucepan and bring to a boil over moderately high heat, stirring until sugar is dissolved and washing down side of pan with a pastry brush dipped in cold water. When syrup reaches a boil, start beating whites with electric mixer at medium-high speed. Once whites are frothy, add lemon juice and beat at medium speed until they just hold soft peaks. (Do not beat again until sugar syrup is readysee below.) Meanwhile, put thermometer into sugar syrup and continue boiling, without stirring, until it reaches soft-ball stage (238242°F). Immediately remove from heat and slowly pour hot syrup in a thin stream down side of bowl into egg whites, beating constantly at high speed. Beat meringue, scraping down bowl with a rubber spatula, until meringue is cool to the touch, about 6 minutes. (It`s important that meringue is fully cooled before proceeding.) 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 butter is added, chill bottom of bowl in a larger bowl filled with ice and cold water for a few seconds before continuing to beat in remaining butter.) Continue beating until buttercream is smooth. (Mixture may look curdled before all butter is added, but will come back together before beating is finished.) Add coconut extract and beat 1 minute more.