Preheat the oven to 350 degrees F. Butter 2 (8-inch) cake pans and line with parchment. Butter the parchment. Sift together the cocoa, baking soda, and 1/2 teaspoon salt into a medium bowl. Into a separate bowl, sift the cake flour.
In the bowl of an electric mixer, beat the brown sugar and butter until very light and fluffy, 5 minutes. Meanwhile, stir together eggs and vanilla in a small bowl. With mixer on medium speed, add the eggs in two batches, scraping bowl occasionally, until incorporated. Beat the batter on high speed 1 minute.
Lower the mixer speed to low; add the sifted cocoa mixture and mix until just combined. Alternately, add flour in three parts with sour cream, beginning and ending with flour. With motor running, gradually add the hot water; beat until just combined. Scrape the bowl thoroughly and beat on low speed for 30 seconds.
Divide the batter evenly between the prepared pans. Bake until the cake pulls away from sides of pan, 30 to 35 minutes. Transfer the cakes to wire racks to cool in pans 30 minutes; remove from pans to cool completely. Meanwhile, make the filling.
To make the filling: In a medium saucepan, combine 2 1/2 cups water, sugar, corn syrup and cocoa. Bring the mixture to a boil over medium-high heat, stirring often. In a small bowl, combine the cornstarch and the remaining 1/2 cup water; stir into cocoa mixture. Return to a boil over medium heat, whisking constantly, boil 4 minutes or until the mixture is very thick and cornstarch is cooked. Remove from heat; stir in the butter and vanilla. Pour into a bowl and cover the surface directly with plastic wrap; cool completely. Refrigerate until chilled, at least 1 hour, before using.
To make the frosting: In a microwave-safe bowl, combine the butter and chocolates; microwave until melted. Stir thoroughly to combine, then set aside. In a medium saucepan, whisk the eggs, sugar, and salt until combined; whisk in water. Cook the egg mixture over medium to medium-high heat until it registers 160 degrees on a candy thermometer, stirring constantly with a whisk (create as little foam as possible). Strain mixture through a fine-mesh sieve into a bowl; whisk in the reserved chocolate mixture. Cool completely; refrigerate until stiff enough to spread, about 2 hours and up to overnight.
To assemble: Cut the tops off the cake layers to make them flat and even on top. Place the bottom layer with trimmed-side facing up on a serving platter; spread blackout filling on top. Place the top layer with trimmed-side facing down on top of the filling. Coat the cake with the frosting.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.