01 - Preheat the oven to 350°F. Grease and flour two 8-inch round cake pans.
02 - Sift together flour, cocoa powder, baking powder, baking soda, and salt in a large bowl.
03 - Whisk mayonnaise, granulated sugar, brown sugar, and vanilla extract in another bowl until smooth and well combined.
04 - Alternate adding the dry ingredients and cold water to the mayonnaise mixture, beginning and ending with dry ingredients. Mix until just combined—do not over-mix.
05 - Divide the batter evenly between the prepared pans, smoothing the tops with a spatula.
06 - Bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean.
07 - Allow cakes to cool in pans for 10 minutes, then invert onto wire racks to cool completely before frosting.