Fudgy and rich, you'd never know there was mayonnaise in this easy, one-bowl chocolate cake. Don't tell your family and see if they can guess the secret ingredient! You can add a frosting if you like, but it really doesn't need more than a dusting of powdered sugar on top.
Technique tip: The cake will rise a lot in the oven, so don't worry if it only looks like a little bit of batter in the pan. And don't use a pan any smaller than 8 inches or it may overflow.
Swap option: if you don't have mayonnaise but do have eggs and oil, use 2 eggs and 1/3 cup oil in place of the mayo.
Heat the oven to 350°F. Grease an 8 or 9-inch cake pan and line the pan with parchment paper, leaving 2 inches of overhang on two of the sides to act as "handles" (this is optional but it helps the cake come out of the pan).2.
Put the cocoa powder and chocolate chips in large bowl and pour the boiling liquid over it. Let sit until the chocolate melts, about 3 minutes. Whisk well.3.
In a medium bowl, whisk together the flour and baking soda and set aside.4.
Whisk the sugar, mayonnaise and salt into the cocoa mixture, then whisk in the flour mixture, whisking vigorously until smooth. Whisk in the vanilla extract or Bourbon.5.
Scrape the batter into the cake pan. Bake until the top of the cake springs back when lightly pressed, 25-40 minutes. The smaller the pan (which means the deeper the batter), the more time it will take to bake.6.
Transfer pan to a wire rack to cool completely. Run a small spatula or butter knife around the edges of the pan to release the cake, then unmold it. Dust with confectioners' sugar before serving if you like.