Preheat the oven to 350°F (180°C) and grease or line an 8-inch (20 cm) square baking tin.
Add the mayonnaise and water to a large mixing bowl and whisk to combine.
1 cup full-fat mayonnaise, 1 cup water
Whisk in the sugar, cocoa, baking soda, and salt until no lumps are visible. Mix in the flour until just combined, being careful not to overmix. Some lumps at this point are fine.
1 cup white sugar, ¼ cup cocoa powder, 2 teaspoons baking soda, ½ teaspoon salt, 2 cups white flour
Transfer the cake batter to the prepared tin and smooth out the top. Bake for 35-40 minutes, or until a toothpick inserted into the middle of the cake comes out clean.
Cool fully before frosting and serving. This cake can be stored for up to 3 days at room temperature and freezes well.