I just made this cake this week - I only do it once a year for my husband's birthday because it's quite time-consuming, but it's absolutely delicious and totally worth it. If you can, use the highest quality chocolate you can find, it's worth it.
All-Chocolate Blackout Cake
1/2 c. unsweetened Dutch-process cocoa powder
4-5 T. boiling water
2 oz. unsweetened chocolate, chopped
3/4 c. milk
1 c. unsalted butter, softened slightly
2 c. sugar
4 large eggs, separated
2 t. vanilla extract
2 c. all-purpose flour
1 t. baking powder
1 t. baking soda
1 t. salt
1 T. plus 1 3/4 t. unsweetened Dutch-process cocoa powder
2 c. boiling water
3/4 c plus 1 T. plus 1/2 t. sugar
1 oz. bittersweet chocolate, chopped
2 T. cornstarch dissolved in 1 T. cold water
1/4 t. salt
1 t. vanilla extract
2 T. unsalted butter
12 oz. semisweet chocolate, chopped (I've used bittersweet for a darker frosting with good results)
12 T. unsalted butter
1/2 c. hot water
1 T. light corn syrup
1 T. vanilla extract
Make the cake:
1. Preheat oven to 350 degrees. Butter and lightly flour 2 8-inch round cake pans (I use parchment paper.)
2. Place cocoa in small bowl and whisk in the boiling water to form a paste.
3. Combine the chocolate and milk in small saucepan over medium heat. Stir frequently until chocolate melts, 3-5 minutes. Remove from heat. Whisk a small amount of milk mixture into cocoa paste to warm it. Whisk cocoa mixture into milk mixture. Return pan to medium heat and stir for 1 minute. Remove and set aside to cool until tepid.
4. Cream butter and sugar together. Beat in egg yolks, one at a time, and add the vanilla. Slowly stir in chocolate mixture
5. Combine the flour, baking powder, soda, and salt. Using a spatula or a wooden spoon, slowly add flour mixture to chocolate mixture. Fold in just until mixed.
6. In another bowl, beat egg whites until soft peaks form. Gently fold egg whites into the batter. Divide batter between prepared pans.
7. Bake until a toothpick inserted in the center of a cake comes out clean. Start checking cakes at 25 minutes -though total bake time is between 30-35. (Better underdone than overdone!!)
Cool cakes in pans on rack for 15 minutes. Gently remove the cakes from pans and continue to cool.
Make the filling:
While the cake is baking, combine the cocoa and boiling water in a small saucepan over low heat. Stir in the sugar and chocolate. Add the dissolved cornstarch paste and salt to the pan and bring to a boil, stirring constantly. Boil for 1 minute. Remove the pan from the heat and whisk in vanilla and butter. Transfer the mixture to a bowl, cover, and refrigerate until cool.
(Note: This makes very runny filling that seeps into the cake - if you like a thicker filling, add an extra tablespoon of cornstarch.)
Make the frosting:
1. Melt the chocolate in a double boiler over hot water, stirring until smooth. Remove the top of hte double boiler from heat and whisk in the butter, 1 T. at a time. Return the top to the heat, if necessary to melt the butter.
2. Whisk in hot water all at once and whisk until smooth. Whisk in corn syrup and vanill. Cover and refrigerat for up to 15 minutes prior to using.
Assemble the cake:
1. Use a sharp serrated knife to slice each cake layer horizontally in half to form 4 layers. Set 1 layer aside. Place 1 layer on a cake round or plate. Generously swath the layer with 1/3 of the filling. Add the second layer and repeat. Set the third layer on top. Quickly apply a layer of frosting to the top and the sides of the cake. Refrigerate cake for 10 minutes.
2. Meanwhile, crumble the remaing cake layer. Apply the remaining frosting to the cake. Sprinkle the cake liberally with cake crumbs. Serve the cake within 24 hours. Store in a cool place.