To make the cakes:

1. Preheat the oven to 350 degrees and spray the insides of two 6 inch round cake pans with a flour based baking spray. Line the bottoms with parchment paper and set aside.
2. Prep the ingredients. Add the 1/2 cup stout to a measuring cup and stir in the cocoa powder, vanilla and espresso powder until the mixture is thick and creamy. Set aside.
3. Add the buttermilk to another measuring cup/glass and vigorously stir in the oil until well blended and smooth. Set aside. 
4. In a large mixing bowl (large enough to hold all of the batter, I used a 3 quart size) combine the egg, egg yolk, baking soda, salt and granulated sugar. Whisk vigorously for one minute until thick and pale. Add the buttermilk/oil mixture and sour cream, and mix well for another minute until well blended and smooth.
5. Add the flour all at once, and whisk for 2 minutes.
6. Pour in the stout/cocoa powder mixture, using a spatula to scrape it all into the cake batter. Add the melted chocolate and mix for one more minute.
7. Divide the cake batter evenly between the two prepared cake pans and bake for 32-35 minutes. The tops of the cakes should be domed and will spring back lightly when touched. Allow the cakes to cool completely to room temperature. 
8. Once the cakes are a cool room temperature, use a large serrated knife to level off the tops. Eat the scraps, they’re delicious!! Using a pastry brush, bathe the cut ends of the cake layers with 2 TB of coffee per layer, for a total of 1/4 cup of coffee used. Wrap the soaked layers in plastic wrap and refrigerate overnight (8 hours).

Make the buttercream and finish the cake:

1. Add the butter to the bowl of a stand mixer and beat on medium high speed for 2 minutes. Scrape down the sides and bottom of the bowl and add the powdered sugar, salt, vanilla, Irish cream liquor and coffee. Mix on low speed until everything is absorbed, then increase the speed to medium high and beat for one minute.
2. Scrape down the sides and bottom of the bowl once more and beat for another minute.
3. Add about a teaspoon of frosting to the surface of the turntable or plate you are using to assemble the cake. Place the first chocolate cake layer onto the dab of frosting to keep it secure. 
4. Add 3/4 cup of frosting to the top of the first layer and spread it evenly over the cake. You should have about 1/2 inch of frosting. Place the second layer on top of the frosting and repeat with another 3/4 cup. Use the remaining frosting to cover the sides of the cake and smooth everything out as desired.
5. If using the ganache, heat the heavy cream in the microwave for 45 seconds on full power. Stir the cream, and then heat for another 15 seconds. Add the chopped chocolate to a heatproof bowl and pour the hot cream on top. Allow the chocolate and cream to rest for one minute, then whisk until smooth.
6. Garnish the frosted cake with the ganache as desired. Serve immediately or chill until you’re ready to eat. Leftover cake can be frozen for 4-6 months or refrigerated for up to a week.