Preheat the oven to 350F. Butter the bottoms and sides of 2 - 8" x 2" round cake pans. Line the bottoms with parchment. Create a parchment collar to go around the sides of the pans.
In a medium bowl, combine the flour, sifted cocoa powder, baking powder and salt. Whisk to combine well.
In a measuring cup, combine the buttermilk and stout. Whisk to combine.
Place the softened butter and the brown sugar into the bowl of a stand mixer. Using the paddle attachment, cream the butter and the brown sugar on medium speed until it is very light in color and super fluffy, about 6-8 minutes.
Add the eggs one at a time, mixing just until combined. Scrape the bottom and sides of bowl with spatula after each addition.
Add one third of the flour mixture to the bowl and mix on low until just combined. Pour half of the beer mixture into the bowl and mix on low until just combined.
Remove the bowl from the mixer. Scrape the bowl to make sure the ingredients are incorporated. The mixture might look curdled at this point, but it will come back together.
Add an additional one third of the flour mixture and mix until just combined. Add the remaining beer mixture and mix just until combined.
Scrape the sides and bottom of the bowl to make sure the ingredients are incorporated.
Add the remaining flour mixture and mix until just combined. Remove the bowl from the mixer. Scrape the sides and bottom of the bowl to make sure there are not any pockets of butter or flour remaining.
Place the bowl back on the stand mixer. With the mixer speed on low add the mayo. Mix for 10 seconds.
Combine the baking soda and vinegar in a small bowl. Pour the mixture into the cake batter and mix on low for an additional 10 seconds.
Remove the bowl from the mixer. Portion the batter evenly into the prepared pans.
Bake the cakes on the center rack of the oven for 40-50 minutes, or until a toothpick inserted in the center comes out with a few crumbs. You will want to start watching for the cakes to be done, when the tops dome and start to crack a bit.
Remove the baked cakes from the oven. If they have domed at all, use a tea towel to gently press down the domes. Let the cakes cool in the pans for 10 minutes.
Run a butter knife around the edge of the pans, remove the parchment collar and turn the cakes out onto a wire rack. Cool to room temperature.
Wrap the cakes tightly with a couple layers of plastic wrap and place in the refrigerator to chill completely before assembling.
The cakes will keep tightly wrapped in plastic wrap and placed in a zip top bag in the refrigerator for up to three days. The cake layers can also be frozen, wrapped well in plastic wrap and placed in a zip top bag. I personally don't like to freeze longer than a week, but you can freeze these layers for up to three weeks. Defrost the layers in the refrigerator before using.
Place the chopped chocolate and butter into a medium size bowl. Set aside.
Heat the heavy cream, salt and corn syrup in a small sauce pan over medium-low heat on the stove until the mixture begins to simmer.
Pour the hot cream over the chocolate. Let the mixture sit for a minute to allow the chocolate to melt. Gently whisk the chocolate and cream until smooth.
Add the Irish whiskey and whisk to combine.
Place the ganache into the refrigerator until it is a thicker, spreadable consistency, like peanut butter, about 30 minutes or so.
Add the salt, espresso and Irish cream to the prepared buttercream. Mix on low until incorporated completely and the buttercream is silky and smooth
Work with chilled cake layers. Using a long serrated knife torte the layers, cutting them evenly in half. You will have four cake layers total. The tops of the cakes will be the center two layers of your cake.
Put one of the cake layers, cut side up onto an 8" cake board or directly onto a cake stand.
Spread a thin, even layer of the ganache on the cake layer with an offset spatula.
Spread about one cup of the buttercream on top of the ganache using an offset spatula to evenly distribute the buttercream. You're looking for the buttercream to be about 1/4 inch thick.
Place another cake layer on top, top up, and press down gently so that the layer is adhered to the buttercream. Repeat the previous two steps, applying the ganache and buttercream.
Place the third layer cake layer, top up, and press down gently so that the layer is adhered to the buttercream.
Add the final ganache layer. Add the final buttercream layer. Place the final cake layer (the bottom of one of the cakes) bottom side up on the buttercream, pressing gently to adhere.
Frost the cake with the remaining coffee Irish cream buttercream. Chill in the refrigerator for 30 minutes.
Microwave the remaining ganache in 3-5 second intervals, stirring after every interval until the ganache is loose and pourable but not hot.
Add the drips around the perimeter of the cake. Fill in the top of the cake with the ganache and smooth.
Slice into pieces and serve.
The completed cake will keep in the refrigerator for up to three days. Bring to room temperature before serving.
To adjust the recipe for different size pans: (Creates 2 - 2" layer cakes to be torted, or you can divide them into three pans)
Halve the recipe for 6" cakes
One and a half recipes for 10" cakes
Two and a half recipes for 12" cakes
Cake layers adapted from The Sweetapolita Bakebook