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1
Center an oven rack and preheat the oven to 350u00b0F.
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2
In a small bowl, whisk together the cocoa and the hot coffee. Blend in the sour cream and vanilla, and set aside.
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3
In another bowl, sift together the flour, baking soda, and salt, then whisk the mixture by hand to ensure that the ingredients are well mixed.
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4
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugars together on medium-high speed until fluffy, about 5 minutes. As you make the batter, stop the mixer frequently and scrape the paddle and the sides of the bowl with a rubber spatula. Blend in the egg yolks and the eggs one at a time, adding the next one as soon as the previous one has disappeared into the batter. With the mixer on low speed, add the flour mixture in three parts, alternating with the cocoa mixture in two parts, beginning and ending with the flour. After each addition, mix until just barely blended and stop and scrape the bowl. Stop the mixer before the last of the flour has been incorporated and complete the blending by hand with a rubber spatula to ensure you do not overbeat the batter.
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5
Divide the thick batter evenly between the two prepared pans (there will be approximately 1 pound 4 ounces per pan) and smooth the tops. Tap the pans on the counter to eliminate any air bubbles. Bake in the middle of the oven until the centers spring back when lightly touched and small cracks have formed on the surface, 35 to 40 minutes. Cool the cakes in their pans on a wire rack for 30 minutes. Flip the cakes out of the pans, leaving on the parchment paper until you assemble the cake. Let them continue to cool on the rack, top sides up, until they reach room temperature.
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6
Make the buttercream:
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7
Using a hand whisk, whisk together the egg whites, sugar, and cream of tartar in the clean bowl of a stand mixer. Place the bowl over (not in) a saucepan of simmering water. The egg white mixture will be gloppy and thick, but as the mixture begins to warm up, it will become more fluid. Continue to gently whisk the mixture until it is very hot to the touch (130u00b0F on a candy thermometer).
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8
Move the bowl to the stand mixer and, using the whisk attachment, whip the whites on medium-high speed until they have tripled in volume and are thick and glossy and hold stiff peaks (like meringue), 3 to 4 minutes. Turn the mixer down to medium-low speed until the mixing bowl is just cool to the touch, 1 to 2 minutes. Kick the mixer back up to medium-high speed and add the butter one piece at a time, adding the next piece just as the previous one has been incorporated. Stop the mixer every so often to scrape down the escaping buttercream from the sides of the bowl. At some point, the buttercream will take on a curdled appearance; don't worry, this is normal. Just keep on mixing until it comes together. Once all the butter is incorporated and the frosting is fluffy and creamy, blend in the vanilla and salt until fully combined.
