Preheat Oven and Prepare Pans: Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans or one 9x13-inch pan.
Combine Dry Ingredients: In a large bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, and salt.
Add Wet Ingredients: Add the mayonnaise, water, and vanilla extract to the dry ingredients. Mix until just combined; do not overmix.
Bake: Pour the batter into the prepared pans. Bake for 20-25 minutes for round pans or 30-35 minutes for a rectangular pan, or until a toothpick inserted into the center comes out clean.
Cool: Allow the cakes to cool in the pans for 10 minutes, then turn them out onto wire racks to cool completely.