NOTE: It is SUPER IMPORTANT that all the wet ingredients listed are room temperature and not cold so that the ingredients mix and incorporate correctly.
Preheat your oven to 350ºF/177º C.
Prepare three 8-inch cake pans with cake goop, parchment paper, or preferred pan release spray. Whisk together the Guinness, espresso powder, and vanilla in a measuring cup and set it aside.
Sift together the cake flour, cocoa powder, salt, baking powder, and baking soda in a large bowl and set it aside.
Place the softened butter in the bowl of a stand mixer with the paddle attachment. Cream the butter on medium speed until it's smooth and shiny. I'm using a stand mixer but you can definitely use an electric mixer if that is what you have.
Gradually sprinkle in the sugar on low speed, then bump it up to medium and cream the butter and sugar until the mixture is fluffy and almost white, about 3-5 minutes.
Add the eggs one at a time. Allow each egg to incorporate fully before adding in the next egg to avoid breaking your batter.
Add in the mayo and mix until combined.
With the mixer on low speed, add ⅓ of the dry ingredients and mix until almost combined. Add ⅓ of the liquid ingredients. Repeat the process with the flour mixture and liquids two more times until everything is fully incorporated.
Gently fold in the mini chocolate chips.
Pour the batter into the prepared pans and bake them for 35-40 minutes until a wooden skewer comes out clean but with a few sticky crumbs.
Let the cake cool in the pan for about 20 minutes then remove them and place them onto a wire rack to cool while you prepare the Bailey's drizzle.