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1
Preheat oven to 350 degrees.
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2
Butter two 9-by-2-inch round cake pans; line bottoms with parchment paper.
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3
Butter parchment; dust with cocoa powder, tapping out excess.
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4
In a medium bowl, whisk cocoa with hot water until smooth.
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5
Whisk in sour cream; let cool.
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6
Into a medium bowl, sift together flour, baking soda, and salt; set aside.
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7
In the bowl of an electric mixer fitted with the paddle attachment, beat butter and sugar on medium speed until light and fluffy, 3 to 4 minutes.
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8
Add eggs, one at a time, beating to combine after each; scrape down sides of bowl as needed.
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9
Beat in vanilla.
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10
With mixer on low speed, add flour mixture in two parts, alternating with cocoa mixture and beginning and ending with flour; beat until combined.
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11
Divide batter between prepared pans; smooth with an offset spatula.
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12
Bake until a cake tester inserted in centers comes out clean, 45 to 50 minutes.
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13
Transfer pans to a wire rack to cool 15 minutes.
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14
Invert cakes onto rack; peel off parchment.
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15
Turn cakes over; let cool completely, topside up.
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16
Transfer half of Chocolate Ganache (3 1/2 cups) to clean bowl of an electric mixer; set aside remaining.
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17
Let cool completely, stirring frequently, about 40 minutes.
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18
Attach bowl to mixer fitted with whisk attachment.
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19
Beat on medium-high speed until ganache holds soft peaks, 5 to 7 minutes.
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20
Using a serrated knife, trim tops of cake layers to make level.
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21
Transfer one of the layers to a cake turntable or platter, and spread top with 1 1/2 cups whipped ganache.
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22
Top with remaining layer, cut side down, and spread remaining whipped ganache in a thin layer over entire cake, covering completely.
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23
Refrigerate until set, about 30 minutes.
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24
Transfer cake to a wire rack set over a rimmed baking sheet.
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25
Pour reserved ganache over top, letting it run down the sides.
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26
If necessary, use a large offset spatula to spread from the center toward the edges, so that the cake is evenly and completely covered.
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27
Refrigerate until ganache has just begun to set, about 30 minutes.
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28
Transfer cake to a serving plate.
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29
Serve immediately, or refrigerate, covered with a cake dome, for up to 2 days.
