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1
Chop chocolate.
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2
In a metal bowl set over a saucepan of barely simmering water melt chocolate, stirring until smooth, and let cool.
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3
Cut butter into pieces and soften to cool room temperature.
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4
In a 1 1/2-quart heavy saucepan bring water and sugar to a boil, stirring until sugar is dissolved.
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5
Boil syrup, without stirring, until a candy thermometer registers 248F.
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6
While syrup is boiling, in a bowl with an electric mixer beat whites with a pinch salt until foamy and beat in cream of tartar.
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7
Beat whites until they just hold stiff peaks and beat in hot syrup in a stream (try to avoid pouring onto beaters and side of bowl).
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8
Beat mixture at medium speed until completely cool, 5 to 10 minutes.
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9
Beat in butter, 1 piece at a time, beating until mixture is thickened and smooth.
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10
(Buttercream will at first appear very thin and at some point look like it is breaking but, as more butter is beaten in, it will thicken and become glossy and smooth.)
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11
Beat in cocoa powder, melted chocolate, and a pinch salt, beating until smooth.
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12
(Buttercream may be made 2 days ahead and chilled in an airtight container.
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13
Bring buttercream to room temperature and beat before using.)
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14
(If buttercream is too cold when beaten it will not be glossy and smooth.)
