Preheat oven to 350°F, and grease and flour two 8-inch cake pans.
In a small bowl, pour the boiling water over the cocoa, and stir until smooth. Set aside.
Sift together the flour, baking soda and salt. Set aside.
In the large bowl of an electric mixer, beat together the mayonnaise and sugar at medium speed until light and fluffy, about 3 minutes. Add the vanilla and cocoa mixture, beating until incorporated.
With mixer at low speed, gradually add the flour mixture, beating just until batter is combined.
Pour batter into prepared pans, and bake 25 to 30 minutes or until a cake tester inserted in center comes out clean. Cool 10 minutes; remove from pans to wire racks. Allow to cool completely before frosting.