Heat oven to 350°F. Grease and flour a 9×13-inch pan or line with parchment.
Whisk flour, sugar, cocoa, baking soda, baking powder, and salt in a large bowl.
Stir mayonnaise, vanilla, and hot coffee (or hot water) in a separate bowl until smooth.
Pour the wet mixture over the dry ingredients. Mix gently until just combined and no dry streaks remain.
Scrape batter into the prepared pan and smooth the top.
Bake 32–36 minutes, until a toothpick in the center comes out with a few moist crumbs.
Cool on a rack 15 minutes, then turn out or leave in pan to cool completely before frosting or dusting with sugar.