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Combine the chocolate and 1/2 cup of butter and melt (but do not over heat) in a double boiler. Set aside to cool to room temperature.
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Combine the sugar and water in a small saucepan and bring to a boil.
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Continue to boil over medium heat without stirring until the mixture reaches 240 degrees on a candy thermometer.
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Meanwhile beat together the egg whites, cream of tartar, vanilla extract and salt to stiff peaks.
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Slowly pour in the sugar syrup in a thin stream, hitting the side of the bowl and not the beaters. Continue beating the frosting for 10-15 minutes until the meringue is completely cool.
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When completely cool, slowly begin to add the butter a few tablespoons at a time. Beat until smooth after each addition.
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Add the melted chocolate to the meringue mixture in about 4 additions and beat well after each addition.
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Continue beating until frosting reaches a spreadable consistency. This may take 15 minutes or so and you may have to chill the frosting for several minutes before beating it again to get it to stiffen. Alternatively, you can place the frosting bowl in an ice bath while continuing to beat constantly until the icing stiffens sufficiently.