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1
Preheat oven to 350u00b0 F. Butter and paper three 8-inch round cake pans.
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2
Melt the chocolate in a double-boiler or a small bowl set atop (but not in contact with) a saucepan of simmering water. Set aside to cool slightly.
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3
Separate the egg yolks from the whites, setting both aside.
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4
In a medium bowl, whisk together the flour, baking soda, baking powder, and salt. Set aside.
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5
In a small bowl or 2-cup glass measuring cup, whisk together the sour cream and vanilla. Set aside.
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6
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar until fluffy and lighter in color, about 3 minutes.
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7
Whisk the egg yolks to break them up, then add them to the batter, beating at medium speed for 2 minutes. Stop the mixer and scrape the sides of the bowl, then beat in the melted chocolate until combined.
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8
Turn the mixer to low and alternate adding the flour, baking soda, baking powder, and salt, and the sour cream in three total parts, beginning and ending with the dry ingredients. Stop when there are still a few streaks of flour left and fold by hand until all is incorporated.
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9
In the very clean bowl of a stand mixer or a separate bowl and with a hand mixer, beat the egg whites until stiff peaks. Gently fold the egg whites into the cake batter.
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10
Divide the batter evenly between the three pans. Bake for 25 to 30 minutes, or until the cakes start pulling away from the sides of the pan and a toothpick inserted in the center comes out clean or with just a few dry crumbs attached. Let cool in the pans for 5 minutes before removing the cakes to racks to cool completely.
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11
In a saucepan, gently stir together the sugar and water. Place over medium heat and cover the pan. Let cook for three minutes, then remove the lid. Gently swirl the mixture and continue to cook at a low boil until it reaches 235u00b0 F on a candy or instant read thermometer.
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12
While the sugar syrup is cooking, whip the egg whites in the bowl of a stand mixer fitted with the whisk attachment until they hold medium peaks.
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13
Once the syrup is done, carefully and slowly stream it into the mixer (while running it on low speed), trying your best to avoid both the whisk and the sides of the bowl. Once all of the syrup is in, turn mixer speed to medium-high and beat until the bowl is cool, and the frosting holds its shape and is a good, spreadable consistency. Whisk in the citric acid or lemon juice on low speed.
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14
To assemble the cake, spread a little of the icing between each cooled layer, then cover the cake completely with the remaining icing.
