Step 1Heat the oven to 350 degrees. Grease and lightly flour 2 (8-inch) round cake pans.
Step 2Stir together the cake flour, sugar, cocoa, salt and baking soda, and sift the mixture onto a piece of wax paper.
Step 3In a bowl beat together the mayonnaise, coffee and vanilla until thoroughly blended. Add the combined dry ingredients and beat until the batter is perfectly smooth. Divide the batter evenly between the prepared pans and bake until a toothpick inserted in the center of the cake comes out clean, about 30 minutes.
Step 4Remove the cakes from the oven and let cool in the pans for 5 minutes before turning them out onto racks to cool completely. Dust with powdered sugar.