16tablespoons2 sticks unsalted butter, softened but still cool
1 1/2cupspackeddark brown sugar
3large eggsroom temperature
16ouncessemisweet chocolateI used Ghiradelli
1 1/2cupsheavy cream
1/3cuplight corn syrup
Adjust the oven racks to the upper- and lower-middle positions and preheat the oven to 350° F. Grease two nine-inch cake pans with butter and line the bottom of each pan with a round of parchment paper or waxed paper. (or use Miracle Pan Release)
Combine the chocolate and cocoa in a medium bowl. Pour the boiling water over and whisk until smooth. In a separate bowl, whisk together the flours, baking soda and salt. Set aside.
Beat the butter in the bowl of a standing mixer at medium-high speed until creamy, about one minute. Add the brown sugar and beat at high speed until light and fluffy, about three minutes. Stop the mixer and scrape down the bowl with a rubber spatula.
With the mixer at medium-high speed, add the eggs one at a time, beating 30 seconds after each addition. Reduce the speed to medium, add the sour cream and vanilla and beat until combined, about 10 seconds. Stop the mixer and scrape down the bowl.
With the mixer on low speed, add about a third of the flour mixture, followed by about half of the chocolate mixture. Repeat, ending with the flour mixture. Beat until just combined, about 15 seconds. Do not overbeat. Remove the bowl from the mixer, scrape the bottom and sides of the bowl with a rubber spatula and stir gently to thoroughly combine.
Divide the batter evenly among the cake pans and smooth the batter to the edges of each pan with a rubber spatula. Place two pans on the lower middle rack. Bake the caked until a toothpick or skewer inserted in the center comes out clean, 20 to 23 minutes. Cool the cakes on wire racks 15 to 20 minutes. Run a knife around each pan perimeter to loosen. Invert each cake onto a large plate, peel off the parchment paper and reinvert onto the rack. Cool completely.
Place the chocolate in a heatproof bowl. Bring the heavy cream to a simmer in a small saucepan over medium-high heat; pour over the chocolate. Add the corn syrup and let stand 3 minutes. Whisk gently until smooth. Stir in the vanilla.
Refrigerate 1 to 1 1/2 hours, stirring every 15 minutes, until the mixture reaches a spreadable consistency. (Note: This frosting does not keep well, so it should be served within a day).
Use a long serrated knife level the two cakes. Place one of the layers on a serving plate. Spread about one cup filling over the cake half. Place second layer on top and use the rest of the frosting to frost the top and sides of the cake. Cut the cake into slices and serve.