1. Preheat the oven to 350°F and grease two 9-inch round cake pans, then line them with parchment paper.

2. In a large bowl, whisk together the flour, cocoa powder, sugar, baking soda, and salt.

3. In another bowl, whisk together the mayonnaise, water, vanilla extract, and eggs until smooth.

4. Pour the wet ingredients into the dry ingredients and mix just until the batter is smooth.

5. Divide the batter evenly between the prepared pans and bake for 28 to 32 minutes, or until a toothpick comes out with a few moist crumbs.

6. Let the cakes cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.

7. For the frosting, beat the butter until creamy, then mix in the cocoa powder and powdered sugar.

8. Add the heavy cream and vanilla extract, then beat until the frosting is smooth and fluffy.

9. Place one cake layer on a plate, spread frosting on top, add the second layer, and frost the top and sides.

10. Slice and serve once the frosting is set.