“I thought it was the best cake I’ve ever had in my life. The mayo made it so moist it melted in my mouth!” —Henry, recipe tester, age 9
Adjust oven rack to middle position and heat oven to 350 degrees. Spray inside bottom and sides of 8-inch square metal baking pan with vegetable oil spray. Line bottom of baking pan with 8-inch square piece of parchment paper.
In medium bowl, whisk together flour, sugar, baking soda, and salt. In large bowl, combine cocoa and chocolate chips.
Heat water in liquid measuring cup in microwave until hot and steaming, 1 to 2 minutes. Use oven mitts to remove measuring cup from microwave. Carefully pour water over chocolate mixture and whisk until smooth. Let mixture cool for 10 minutes.
Add mayonnaise, egg, and vanilla to cooled chocolate mixture and whisk until combined. Add flour mixture and use rubber spatula to stir until just combined and no dry flour remains.
Use rubber spatula to scrape batter into parchment-lined baking pan and smooth top (make sure to spread batter into corners to create even layer).
Place baking pan in oven. Bake until toothpick inserted in center of cake comes out clean, 34 to 38 minutes.
Use oven mitts to remove baking pan from oven (ask an adult for help). Place baking pan on cooling rack and let cake cool completely in pan, about 2 hours.
Remove cake from baking pan and discard parchment (click here to learn how to remove a cake from its baking pan). Dust cake with confectioners’ sugar. Cut cake into pieces and serve.
Using mayonnaise as an ingredient in this recipe seemed strange to us at first, too. But then we remembered that mayonnaise is mostly made from two common cake ingredients: egg yolks and oil! (Mayo also contains an acid such as vinegar or lemon juice.) Two-thirds of a cup of mayonnaise is the trick to making this cake super moist and delicious. And you might already have this key ingredient in your refrigerator or pantry.
Even though this picture shows our Raspberry Clafouti, the technique is the same. Here’s how to do it: