Heat your water. I usually put my tea kettle on while I'm getting the other ingredients ready in the bowl and it's nice and hot by the time I need it.
In a large bowl, sift flour, sugar, and cocoa powder adding them one on top of the other.
Add in baking soda, salt, vanilla extract, and mayonnaise.
Pour hot water over ingredients.
Stir until all the batter is mixed together and you don't have any flour streaks. Flour may get a little bit lumpy, don't worry about it.
Pour batter into greased 9x9 pan. Or you can use a 9 inch round or a 8 inch round as long as they are at least 2 inches tall.
Bake at 350 degrees for 35 minutes or until you insert a toothpick and it comes out with just crumbs. Chocolate cakes are tricky to try and tell when they are done because you can't see how brown they are getting so it's easy to over bake them. I usually start checking mine at 30 minutes.
Cool on a cooling rack for 15 minutes and then turn out of the pan. Allow to cool for at least 1-2 hours before frosting. I like to refrigerate my cakes overnight before frosting because I think frosting a cold cake makes life a whole lot easier.