Mile-High Chocolate Cake with Vanilla Buttercream

Butter three 8-inch-diameter cakepans with 1 1/2-inch-high sides. Line bottomof each pan with parchment paper round;butter parchment. Dust pans with flour; tapout excess.

Whisk 11/4 cups hot water, cocoapowder, and sour cream in medium bowluntil smooth; set aside until mixture is cool,20 to 30 minutes.

Position rack in center of oven andpreheat to 325°F. Sift next 4 ingredientsinto another medium bowl. Using electricmixer, beat butter and shortening in largebowl until well blended and smooth.

Addboth sugars; beat until light and fluffy, about5 minutes.

Add eggs 1 at a time, beatinguntil well blended after each addition.

Mixin vanilla.

Add flour mixture in 3 additionsalternately with cocoa mixture in 2 additions,beginning and ending with flour mixture andbeating just until incorporated. Divide batteramong prepared pans (about 23/4 cups batterfor each) and smooth tops.

Bake cakes until tester inserted intocenter comes out clean, 50 to 55 minutes.Cool cakes in pans on racks 20 minutes.Run knife around sides of cakes to loosen.Invert cakes onto racks and cool completely.DO AHEAD: Can be made 1 day ahead. Wrap inplastic and store at room temperature.

Combine egg whites and 1/3 cup sugar in large bowl ofstand mixer fitted with whisk attachment;scrape in seeds from vanilla bean.

Combine remaining 1 1/3 cups sugar and1/3 cup water in medium saucepan. Stir overmedium heat until sugar dissolves. Attachcandy thermometer to side of pan, makingsure that bulb of thermometer is immersed insyrup. Increase heat; boil until temperatureregisters 238°F to 240°F (soft-ball stage),about 5 minutes. Immediately remove syrupfrom heat (use within 2 minutes).

Meanwhile, beat egg white mixture onmedium speed until very soft peaks form(mixture will be slightly opaque).

Increase mixer speed to high and slowlypour hot syrup down side of bowl intoegg white mixture in slow steady stream;beat until meringue forms stiff peaks. Coolmeringue in bowl until lukewarm (instant readthermometer will register 100°F), about30 minutes. Do not beat.

Start beating meringue again onmedium speed of stand mixer. Gradually addbutter, 2 to 3 tablespoons at a time, beatingconstantly until absorbed before adding nextaddition. Continue beating until buttercreamis smooth. (If buttercream looks broken orcurdled, place mixer bowl with buttercreamover medium heat on stove burner andwhisk with large whisk for 5 to 10 secondsto warm mixture slightly, then remove fromheat and whisk vigorously or attach bowlto stand mixer and beat mixture again onmedium speed. Repeat warming and beatingbuttercream mixture as many times asneeded until buttercream is smooth and nolonger curdled.)

Using serrated knife, cut each cake inhalf horizontally, forming 6 cake layers. Ifnecessary, trim any domed tops of cakesto form even layers.

Place 1 cake layeron platter. Drop 3/4 cup buttercream bytablespoonfuls over top of cake layer, thenspread evenly to edges with offset spatula.Top with second cake layer, then spread 3/4cup buttercream over. Repeat with 3 morecake layers, spreading 3/4 cup buttercreamover each and pressing slightly to adhere.Top with 6th cake layer (do not spreadbuttercream over top cake layer). Chill cakeat least 1 hour.

Combine bothchocolates in medium bowl. Bring cream andcorn syrup to simmer in heavy saucepan.

Pour hot cream mixture over chocolate; letstand 2 minutes.

Whisk chocolate mixtureuntil melted and smooth.

Add butter;whisk until melted. Chill glaze until slightlythickened and glaze drips thickly whenpoured slowly from spoon, 30 to 45 minutes.

Spoon glaze around top edge of cake byteaspoonfuls, spacing drips up to 2 inchesapart and allowing glaze to drip slowly downsides of cake. Spoon remaining glaze overtop center of cake and smooth with offsetspatula, covering top completely. Chill cakeuntil glaze sets, at least 1 hour. DO AHEAD:Can be made 1 day ahead. Tent with foiland keep chilled.

Let cake stand at roomtemperature 1 hour before serving.