• Preheat the oven to 350F. Grease 2 8x2-inch round cake pans, line with parchment, grease again and dust with flour.
• Combine the flour, baking powder, baking soda, salt and cocoa powder in a bowl and whisk for at least a minute or until very well combined.
• In a mixing bowl, whisk together the brown sugar, egg, egg yolk, sour cream, oil, vanilla, and ¾ cup hot coffee (or water) until smooth. Add the dry ingredients and beat by hand with a heavy duty scraper until blended.
• Divide batter between prepared pans. Bake cake until a tester inserted into the center comes out clean, about 30 minutes. Transfer pans to a wire rack; let cakes cool in pan. Invert onto a plate, then invert again.

Pudding Mixture

• In a large saucepan, combine sugar, cocoa powder, corn syrup and 1 ¼ cups of water. Bring to a boil over medium-high heat, whisking occasionally.
• Meanwhile, in a small bowl, whisk the remaining ¼ cup of water and the cornstarch. Whisk the cornstarch mixture into the cocoa mixture in the saucepan and return the mixture to a boil, whisking constantly. Cook, whisking constantly, until thickened, 3 to 4 minutes. Remove from heat and whisk in butter and vanilla.
• Pour into a bowl and whisk for a few minutes to help it cool. Cover with plastic wrap, lightly pressing the plastic against the surface to prevent a skin from forming. Let cool at room temperature for about 20 minutes, then chill until it thickens a bit (45 minutes).

Assembling Cake

• Using a long serrated knife, cut the cake layers in half horizontally. Reserve the worst looking layer for crumbs.
• Place a cake layer on a flat service. If you are planning freezing, you may want to do this on a big sheet of plastic wrap. Spread with half of the cooled pudding. Stack on a second layer of cake, remaining pudding, then the final layer of cake. Wrap the cake tightly in plastic and freeze overnight. If you don't want to freeze overnight, just wrap and chill for a few hours.
• On assembly day, use the frosting below to cover sides and top. Crumble the reserved caked layer and put crumbs all over the top and sides. Chill for a few hours if you can to let everything meld.

Frosting - Allow 2 hours for this. I only needed about ⅔ of the recipe

• Bring cream, syrup, and salt to a boil in a saucepan over medium-high heat.
• Remove from heat and add chocolate. Stir until the chocolate melts, then stir in the room temperature sour cream, and butter. You can add vanilla if you like.
• Transfer to a medium bowl. Let frosting sit at room temperature for about 1 ½ hours, stirring every 30 minutes or so. Stir to combine cooler outer edges with warmer center, then chill another 5 minutes if you need to (I did not). Use to frost the cake.