Mile High Chocolate Cake with Vanilla Buttercream

To prepare the cake:

1. Butter three 8-inch cake pans. Line each pan with a round piece of parchment paper cut to the diameter of the bottom of the pan. Butter the parchment papers, and place in the bottom of each pan. Lightly dust pans with flour.
2. In a medium bowl, whisk 1 1/4 cups of hot water, cocoa powder and sour cream until smooth. Allow mixture to cool, about 30 minutes.
3. Preheat an oven with a rack in the middle position to 325 degrees F.
4. In a second medium bowl, sift the flour, baking powder, baking soda and salt. Set aside.
5. In the bowl of a stand mixer fitted with a paddle attachment, beat butter and shortening until smooth. Add sugars, and beat until fluffy, about 5 minutes. Add eggs one at a time, allowing each to fully blend into the mixture. Blend in vanilla.
6. Add half the flour mixture followed by half the cocoa mixture. Add the remaining flour mixture then the remaining cocoa mixture. Beat until combined.
7. Divide cake batter among the prepared pans. Using an offset spatula, smooth the tops of the cakes.
8. Bake cakes until a cake tester comes out clean, about 50 minutes.
9. Cool the cakes in their pans on cooling racks, about 20 minutes. Run a knife around the edges of the cakes to loosen them from the pans. Invert cakes onto the racks, and allow to cool completely.

To prepare the vanilla buttercream:

1. In the bowl of a stand mixer fitted with the whisk attachment, combine egg whites and 1/3 cup sugar. Scrape in the seeds from a vanilla bean pod.
2. Beat egg white mixture on medium speed until soft peaks form.
3. In a medium saucepan over medium heat, combine remaining 1/3 cup sugar and 1/3 cup water until the sugar dissolves.
4. Attach a candy thermometer to the side of the pan. Increase heat, boiling the syrup until it reaches 240 degrees F, about 5 minutes. Immediately remove syrup from heat.
5. Beat the egg white mixture on high speed, and steadily pour hot syrup down the sides of the bowl. Beat until stiff peaks form.
6. Allow the meringue to cool in its bowl, 30 minutes.
7. Begin to beat the cooled meringue on medium speed. Add butter 2-3 tablespoons at a time, allowing each batch of butter to fully combine into the mixture. Continue beating until buttercream is smooth.

To assemble the cake:

1. Using a cake cutter, cut each cake in half horizontally.
2. On a serving platter, place one cake layer. Spoon 3/4 cup of the buttercream onto the layer. Using an offset spatula, smooth the buttercream evenly over the cake layer.
3. Repeat the cake-then-buttercream stacking process with remaining cake layers, pressing each cake layer lightly onto its layer of buttercream below. Do not add buttercream to the top layer of the cake.
4. Chill the cake at least 1 hour.

To prepare the chocolate glaze:

1. In a medium bowl, combine the milk chocolate and bittersweet chocolate.
2. In a heavy-bottomed saucepan, bring the cream and corn syrup to a simmer.
3. Pour the hot cream mixture over the chocolate. Let stand 2 minutes.
4. Whisk the chocolate mixture until the chocolates have melted and the mixture is smooth.
5. Add butter, and whisk until it has melted into the chocolate.
6. Chill glaze until slightly thickened, about 45 minutes.

To finish the cake:

1. Spoon several teaspoons of the chocolate glaze around the top edge of the cake. Allow some of the glaze to drip down the sides of the cake.
2. Using an offset spatula, smooth the top and sides of the cake until evenly covered.
3. Chill the glazed cake at least 1 hour.
4. Let the chilled cake stand at room temperature for 1 hour before serving.