Now don't freak out. I know this doesn't sound right. Chocolate - mayo - cake?? Did I make a typo up there?? No sir ma'am, I did not. Think about it, what is mayo made of? Eggs, oil, vinegar. What makes a cake moist and delicious? Eggs, oil, vinegar. I'm telling you, this was THE BEST chocolate cake I have ever made. From here on out, mayo is going in ALL of my cakes. And if you hate the condiment, you would never know it. It doesn't taste like mayo, you silly billies, it just makes the cake ridiculously moist and delicious, and full of rich chocolate flavor.
Oh, and also, the chocolate buttercream frosting. I mean, shut up, just shut up recipe with your PERFECT chocolate buttercream frosting. It's absurd. Warning! There are 3 sticks of butter in it, but, if you splurge a little and use the unsalted European kind (found in your regular market) you will not regret it. You will stick your face in it. It's just a wonderful, wonderful cake.
I used this recipe, so there you go.