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1
Butter three 8-inch-diameter cake pans with 1 1/2-inch-high sides.
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2
Line bottom of each pan with parchment paper round; butter parchment.
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3
Dust pans with flour; tap out excess.
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4
Whisk 11/4 cups hot water, cocoa powder, and sour cream in medium bowl until smooth; set aside until mixture is cool, 20 to 30 minutes.
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5
Position rack in center of oven and preheat to 325F.
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6
Sift next 4 ingredients into another medium bowl.
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7
Using electric mixer, beat butter and shortening in large bowl until well blended and smooth.
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8
Add both sugars; beat until light and fluffy, about 5 minutes.
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9
Add eggs 1 at a time, beating until well blended after each addition.
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10
Mix in vanilla.
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11
Add flour mixture in 3 additions alternately with cocoa mixture in 2 additions, beginning and ending with flour mixture and beating just until incorporated.
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12
Divide batter among prepared pans (about 23/4 cups batter for each) and smooth tops.
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13
Bake cakes until tester inserted into center comes out clean, 50 to 55 minutes.
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14
Cool cakes in pans on racks 20 minutes.
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15
Run knife around sides of cakes to loosen.
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16
Invert cakes onto racks and cool completely.
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17
DO AHEAD: Can be made 1 day ahead.
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18
Wrap in plastic and store at room temperature.
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19
Combine egg whites and 1/3 cup sugar in large bowl of stand mixer fitted with whisk attachment; scrape in seeds from vanilla bean.
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20
Combine remaining 1 1/3 cups sugar and 1/3 cup water in medium saucepan.
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21
Stir over medium heat until sugar dissolves.
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22
Attach candy thermometer to side of pan, making sure that bulb of thermometer is immersed in syrup.
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23
Increase heat; boil until temperature registers 238F to 240F (soft-ball stage), about 5 minutes.
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24
Immediately remove syrup from heat (use within 2 minutes).
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25
Meanwhile, beat egg white mixture on medium speed until very soft peaks form (mixture will be slightly opaque).
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26
Increase mixer speed to high and slowly pour hot syrup down side of bowl into egg white mixture in slow steady stream; beat until meringue forms stiff peaks.
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27
Cool meringue in bowl until lukewarm (instant read thermometer will register 100F), about 30 minutes.
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28
Do not beat.
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29
Start beating meringue again on medium speed of stand mixer.
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30
Gradually add butter, 2 to 3 tablespoons at a time, beating constantly until absorbed before adding next addition.
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31
Continue beating until buttercream is smooth.
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32
(If buttercream looks broken or curdled, place mixer bowl with buttercream over medium heat on stove burner and whisk with large whisk for 5 to 10 seconds to warm mixture slightly, then remove from heat and whisk vigorously or attach bowl to stand mixer and beat mixture again on medium speed.
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33
Repeat warming and beating buttercream mixture as many times as needed until buttercream is smooth and no longer curdled.)
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34
Using serrated knife, cut each cake in half horizontally, forming 6 cake layers.
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35
If necessary, trim any domed tops of cakes to form even layers.
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36
Place 1 cake layer on platter.
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37
Drop 3/4 cup buttercream by tablespoonfuls over top of cake layer, then spread evenly to edges with offset spatula.
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38
Top with second cake layer, then spread 3/4 cup buttercream over.
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39
Repeat with 3 more cake layers, spreading 3/4 cup buttercream over each and pressing slightly to adhere.
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40
Top with 6th cake layer (do not spread buttercream over top cake layer).
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41
Chill cake at least 1 hour.
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42
Combine both chocolates in medium bowl.
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43
Bring cream and corn syrup to simmer in heavy saucepan.
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44
Pour hot cream mixture over chocolate; let stand 2 minutes.
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45
Whisk chocolate mixture until melted and smooth.
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46
Add butter; whisk until melted.
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47
Chill glaze until slightly thickened and glaze drips thickly when poured slowly from spoon, 30 to 45 minutes.
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48
Spoon glaze around top edge of cake by teaspoonfuls, spacing drips up to 2 inches apart and allowing glaze to drip slowly down sides of cake.
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49
Spoon remaining glaze over top center of cake and smooth with offset spatula, covering top completely.
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50
Chill cake until glaze sets, at least 1 hour.
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51
DO AHEAD: Can be made 1 day ahead.
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52
Tent with foil and keep chilled.
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53
Let cake stand at room temperature 1 hour before serving.
