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Combine the water and sugar 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 and salt to stiff peaks.
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Slowly pour in the sugar syrup hitting the side of the bowl and not the beaters.
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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.
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Combine the chocolate and whipping cream a double boiler to melt. Be careful to bring it just to the melting point, don't overheat it.
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Add to the meringue mixture and beat well 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.