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1
Melt chocolate in a double boiler or a metal bowl set over a saucepan of simmering water, stirring.
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2
Cool.
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3
Combine egg whites and salt in a large bowl.
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4
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.
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5
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.
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6
(Do not beat again until sugar syrup is ready.)
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7
Meanwhile, put candy thermometer into sugar syrup and continue boiling until syrup reaches 238242F.
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8
Immediately remove from heat and pour into a heatproof 1-cup glass measure.
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9
Slowly pour hot syrup in a thin stream down side of bowl into egg whites, beating constantly at high speed.
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10
Beat, scraping down bowl with a rubber spatula, until meringue is cool to the touch, about 6 minutes.
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11
(Its important that meringue is properly cooled before proceeding.)
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12
With mixer at medium speed, gradually add butter 1 piece at a time, beating well after each addition until incorporated.
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13
(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.)
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14
Continue beating until buttercream is smooth.
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15
(Mixture may look curdled before all butter is added but will come back together before beating is finished.)
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16
Add vanilla, chocolate, and coffee mixture and beat 1 minute more.
