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1
Preheat the oven to 350F, with racks in the upper and lower thirds.
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2
Cut 2 8-inch rounds of parchment paper to line the bottoms of 2 8-inch (2 inches deep) round cake pans.
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3
Butter and line the pans.
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4
Butter the linings.
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5
Dust the pans with cocoa, tapping out excess.
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6
Set aside.
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7
Sift together the cocoa, flour, sugar, baking soda, baking powder, and salt into the bowl of an electric mixer.
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8
Return the bowl to the mixer.
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9
Fit the mixer with the paddle attachment.
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10
Add the eggs, yolk, warm water, buttermilk, oil, and vanilla.
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11
Mix on low speed until smooth, about 3 minutes.
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12
Divide the batter evenly between the prepared pans.
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13
Bake, switching the positions of the pans and rotating them halfway through, until a cake tester inserted into the centers comes out clean, about 40 minutes.
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14
Let cool in the pans on wire racks 30 minutes.
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15
Run a thin knife around the edges of the cakes to loosen.
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16
Unmold; peel off the parchment.
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17
Let cool completely, top sides up, on the racks.
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18
Trim the tops of the cake layers with a long serrated knife to make them level.
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19
Using a small offset spatula, cover the top of one cake layer evenly with 1 1/2 cups frosting; spread the frosting so it extends just beyond the edges of the cake.
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20
Place the second layer, cut side down, on top; press gently to level.
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21
Frost the top and sides of the assembled cake with the remaining frosting.
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22
Put the sugar in the bowl of a food processor, and scrape in the vanilla seeds.
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23
Process until combined.
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24
Sift into a bowl, and discard any large pieces of vanilla.
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25
Put the egg whites, vanilla-sugar mixture, and salt in the heatproof bowl of an electric mixer.
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26
Set over a pan of simmering water.
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27
Whisk constantly until the sugar has dissolved and the mixture registers 160F on an instant-read thermometer.
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28
Return the bowl to the mixer; fit the mixer with the whisk attachment.
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29
Beat on medium-high speed until stiff, glossy peaks form and the mixture is cool, about 10 minutes.
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30
Switch to the paddle attachment.
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31
With the mixer on medium speed, add the butter, 1 or 2 tablespoons at a time, mixing well after each addition.
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32
Mix, scraping down the sides of the bowl as needed, until the mixture is smooth.
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33
Mix in the vanilla extract.
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34
Use immediately, or cover and refrigerate.
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35
Put the sugar, egg whites, and salt in the heatproof bowl of an electric mixer.
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36
Set over a pan of simmering water.
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37
Whisk constantly until the sugar has dissolved and the mixture registers 160F on an instant-read thermometer.
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38
Return the bowl to mixer; fit the mixer with the whisk attachment.
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39
Beat on medium-high speed until stiff, glossy peaks form, and the mixture is cool, about 10 minutes.
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40
Switch to the paddle attachment.
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41
With the mixer on medium speed, add the butter, 1 or 2 tablespoons at a time, mixing well after each addition.
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42
Mix, scraping down the sides of the bowl as needed, until the mixture is smooth.
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43
Mix in the chocolate.
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44
Use immediately, or cover and refrigerate.
