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