Preheat the oven to 350 degrees F.
Sift together the flour, cocoa, and baking soda in a small bowl
In a separate bowl, mix together the sugar and mayonnaise, then beat in the water and vanilla.
Gradually add in the dry mixture and mix well.
Pour the batter into the prepared pans and bake for 20-25 minutes or until a toothpick inserted in the center of the cake
comes out clean.
Make the frosting while the cake is baking.
In a work bowl with an electric mixer, cream the butter. Sift the sugar and cocoa powder together and gradually add to the butter, beating well after each addition. Add the cream and vanilla. Beat until well blended and fluffy.
Remove the cake from the oven, remove the layers from the pans, and place them on wire racks to cool before frosting.