For the Cake: (though the original recipe yields two 9 inch cakes, I have tall 6 inch pans, so I was able to get two very tall 6 inch cakes with the same batter)
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4 oz unsweetened chocolate, finely chopped
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¼ cup (3/4 oz) Dutch-process cocoa powder
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½ cup hot water
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1¾ cups (12¼ oz) sugar
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1¾ cups (8¾ oz) all-purpose flour
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1½ teaspoons baking soda
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1 teaspoon salt
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1 cup buttermilk, at room temperature
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2 teaspoons vanilla extract
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4 large eggs plus 2 large egg yolks, at room temperature
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12 tablespoons (1½ sticks) unsalted butter, cut into 12 pieces, at room temperature