Preheat the oven to 350°F with a rack in the center. Lightly grease a 9" x 13" pan. , In a large bowl, whisk together the mayonnaise, water, and vanilla to dissolve the mayonnaise and form a thin, homogeneous mixture. , Sift in the flour, sugar, cocoa, baking soda, espresso powder, and salt. Whisk to combine. Pour the thick batter into the prepared pan and spread it into an even layer.   , Bake the cake for 30 to 35 minutes, until a toothpick or thin knife inserted into the center comes out clean, or with a few moist crumbs clinging to it.   , Remove the cake from the oven and let it cool in the pan on a wire rack. , Once completely cool, finish the chocolate mayonnaise cake with your frosting of choice (see “tips,” below, for ideas)., Store leftover cake, well covered, at room temperature for several days; freeze for longer storage.