1. Adjust oven rack to middle position and heat oven to 350°F. Lightly spray 8-inch-square baking dish with nonstick cooking spray.

2. Whisk flour, sugar, baking soda, and salt together in large bowl. In separate bowl, combine cocoa and chocolate; pour hot coffee over cocoa mixture and whisk until smooth; let cool slightly. Whisk in mayonnaise, egg, and vanilla. Stir mayonnaise mixture into flour mixture until combined.

3. Scrape batter into prepared pan and smooth top. Bake until wooden skewer inserted into center of cake comes out with few crumbs attached, 30 to 35 minutes.

4. Let cake cool in pan on wire rack, 1 to 2 hours. Dust with confectioners sugar, cut into squares, and serve straight from the pan; or turn cake out onto serving platter and dust with confectioners sugar.

Makes one 8-inch-square cake