Preheat oven to 350 degrees F. Grease pans with my cake pan release and line bottom with parchment paper.
Add all dry ingredients (flour, sugar, dutch processed cocoa powder, black cocoa, baking soda, baking powder, and salt) to a stand mixer bowl with paddle attachment. Combine well.
In a separate bowl, combine eggs, buttermilk, vanilla extract, butter vanilla, and stout. Set aside.
Dissolve espresso powder in hot water. Set aside.
With the mixer on low speed, add the unsalted butter 1 tablespoon at a time into the flour mixture. Keep adding each butter piece until all has been incorporated.
With the mixer on low speed, slowly add the liquid mixture to the butter/dry mixture then turn the mixer to medium speed and beat for 2 minutes. Scrape down the sides of the bowl then turn the mixer back to medium speed and beat for another 3 minutes. Scrape down the sides of the bowl.
Add hot water and espresso mixture and mayonnaise. Mix on medium till well combined.
Divide evenly between pans.
Bake for 60 – 80 minutes or until a skewer comes out clean.
Allow to cool in pans for 10 minutes before removing. Then remove the cakes from the pans and place on a rack to finish cooling.