"With a hint of caramel from toasted sugar, Belgian cassonade, or light brown sugar, this Swiss meringue–based chocolate frosting is rich, complex, and none too sweet...."
INGREDIENTS
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6 ounces egg whites (2/3 cup; 170g), from 5 to 6 large eggs
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11 ounces toasted sugar, Belgian cassonade, or light brown sugar (about 1 2/3 cups; 310g); see note
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1 teaspoon (4g) Diamond Crystal kosher salt (plus more if needed); for iodized salt, use half as much by volume or the same weight
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1/4 teaspoon cream of tartar
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16 ounces unsalted butter (4 sticks; 455g), softened to about 65°F (18°C)
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1 teaspoon (5g) vanilla extract (plus more if needed)
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8 to 14 ounces finely chopped dark chocolate, between 70 and 77% cocoa solids (about 1 1/3 to 2 1/3 cups; 225 to 395g)
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1 teaspoon instant espresso powder (optional), dissolved in a few drops of hot water