Preheat the oven to 350℉. Grease and flour two 8-inch cake pans.
Sift the cocoa, flour and baking soda into a medium bowl. Stir in the sugar and salt.
In a large bowl, use an electric mixer to combine the mayonnaise, vanilla and water and mix until smooth. Add in the dry ingredients a little at a time, just until all is combined. Don’t over-mix.
Divide the batter between the two cake pans; bake for 30 minutes, or until a tester inserted into the center of the cake comes out clean. Cool on wire racks.
FROST THE CAKE:
Melt the butter in a large saucepan. Stir in the cocoa. Use an electric mixer to alternately add the powdered sugar and milk, beating to spreading consistency. Mix in the vanilla. Frost as you would traditionally frost a 2-layer cake.
Video
Notes
Use full-fat mayonnaise for best results (I recommend Hellmans/Best Foods).