Melt chocolate in a double boiler or a metal bowl set over a saucepan of simmering water, stirring. Cool. Combine egg whites and salt in a large bowl. Stir together sugar and water in a small heavy saucepan and bring to a boil over moderately high heat, washing down side of pan with a pastry brush dipped in water. When syrup reaches boil, start beating whites with an electric mixer at medium-high speed until frothy, then beat at medium speed until whites just hold soft peaks. (Do not beat again until sugar syrup is ready.) Meanwhile, put candy thermometer into sugar syrup and continue boiling until syrup reaches 238242°F. Immediately remove from heat and pour into a heatproof 1-cup glass measure. Slowly pour hot syrup in a thin stream down side of bowl into egg whites, beating constantly at high speed. Beat, scraping down bowl with a rubber spatula, until meringue is cool to the touch, about 6 minutes. (Its 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. (If meringue is too warm and buttercream looks soupy after some of 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.) 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 vanilla, chocolate, and coffee mixture and beat 1 minute more.