Preheat the oven to 350°F. Grease and flour three 8" or two 9" cake pans, and line them with parchment paper circles, leaving at least 1 inch of extra paper hanging over the sides (you will use these to pull the baked cake layers out of the pans). Grease the parchment paper (you'll be glad that you did this). Be sure your 9" pans are at least 2" deep.
Place the stout beer and butter in a large, heavy saucepan, and bring to a simmer at medium temperature until the butter melts. Remove the pan from the heat.
Add the cocoa powder and whisk until the mixture is smooth. Set aside to cool to room temperature or warm, yet not hot to the touch.
Whisk together the flour, sugar, baking powder, baking soda, and salt in a large bowl until well combined; set aside.
In the mixing bowl of a stand mixer fitted with the paddle attachment (you can also use a hand mixer), beat together the eggs, sour cream, buttermilk, and vanilla until well blended.
Add the stout-cocoa mixture to the egg-sour cream mixture. Mix until barely combined.
Add the flour mixture and mix together at slow speed. Using a rubber spatula, fold the batter until the flour is completely combined, scraping the sides and bottom of the bowl. Mix again for one more minute. The batter will appear to be very thin.
Pour the batter equally among the prepared pans. Smooth the tops of the cake batter.
Bake the layers for 35 - 44 minutes for 8" pans, or 45 to 50 minutes for 9" pans, until a toothpick or cake tester inserted into the center of the cake comes out clean.
While cakes are baking and cooling, prepare the ganache and Bailey's buttercream.
Remove the cakes from the oven and cool on a wire rack for 10 minutes before turning the cakes out of their pans. Using the parchment paper, pull the cakes out of their pans and cool to room temperature on the wire racks.
For the Ganache and Bailey's Buttercream
Place chopped chocolate in a medium bowl. Heat the heavy cream in a saucepan until simmering, then pour the hot cream over the chopped chocolate and let sit for 4 minutes. Once that time has passed, whisk until a rich, shiny chocolate ganache forms. Allow the ganache to cool until it is thin and spreadable.
Prepare the Bailey's buttercream by creaming the butter in a large bowl or stand mixer fitted with a paddle attachment. Add in the powdered sugar and Bailey's Irish Cream liqueur, a little bit at a time alternating between each. If the buttercream seems too stiff, add a little bit more liqueur.
To Assemble The Cake
Using a very long, serrated knife, slice off the domes of each cake layer so that they will lay flat on top of the platter and to each other when layered.
Frost and serve immediately or wrap tightly in plastic wrap to refrigerate for up to 3 days before frosting and serving.
On a serving platter, first frost the Bailey's buttercream frosting on and refrigerate for 20 minutes, or in the freezer for 10 minutes to harden up the buttercream.
Spoon the ganache all over the chilled buttercream. With a large, angled offset spatula spread the buttercream smoothly over the cake into an even layer or with some vertical lines (see photos).
Pour and spoon the ganache all over the layer of buttercream, Make swoopy swirls in the ganache if you prefer.
