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