I tore out this recipe from Bon Appetit knowing it sounded so wrong it was probably all right. My intuitions were right. The mayonnaise is the perfect replacement for oil and makes for a perfectly moist cake.
This cake is perfect by itself or with ice cream. It keeps well under a dome for a few days, though it may not stick around that long. The frosting is delicious since it's basically butter and sugar- hard to go wrong there.
I had 3 9 inch cake pans but you could just use 2 9 inch or 3 8 inch. For 3 9 inch cakes the cake will cook in less time- for me it was around 20 minutes until a toothpick inserted in the center came out clean. —goodspoon
Preheat the oven to 350. Butter and flour your cake pans and set them aside. Combine the chopped chocolate and cocoa powder in a glass bowl then pour over the boiling water. Whisk together until the mixture is smooth.
Whisk together the flour, baking soda, and baking powder. Using an electric mixer or your beaters, beat both the sugar and mayonnaise until well blended, about 2-3 minutes. Add eggs 1 at a time then beat in vanilla.
Scrape down the bowl then add in flour and chocolate, alternating about 3-4 times starting with the flour. Beat well after each addition and scrape down the sides in the middle and at the end to make sure everything is incorporated. Divide batter among prepared cake pans and bake until a toothpick comes out clean. Start with 20 minutes for 3 9" pans or 25-30 for 8" pans.
When the cakes are done cool them in the pan for 20 minutes then run a knife along the side and invert them onto a cooling racks to cool.
Place chopped chocolate in a medium metal or glass bowl. Place the bowl over a saucepan of simmering water and stir until the chocolate is melted. Remove from the heat and allow the chocolate to cool so it won't melt the butter. Stirring it often helps as you want it to be lukewarm- a good step to do when you place the cakes in the oven or while they are cooling!
Beat the butter in a bowl until smooth and creamy. Add powdered sugar and beat until blended well, about 2 minutes. Beat in vanilla and then add your chocolate and combine until nice and smooth. Scrape down the sides to make sure everything is combined.
Place one cake on a platter (maybe with some wax paper under the edges which you can remove when you're done frosting) and spread about 3/4 cup of the frosting onto the cake, add another cake and another 3/4 cup frosting. Add the final layer and your remaining frosting all around the sides.