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.