7.5 oz unbleached all-purpose flour
7 oz sugar
1/2 teaspoon baking soda
1/4 teaspoon salt
2 oz Dutch-processed cocoa powder
2 oz bittersweet chocolate, chopped fine
1 teaspoon espresso powder
8 oz very hot water
2.25 oz mayonnaise
1 large egg
2 teaspoons vanilla extract
1. Adjust oven rack to middle position and heat to 350 degrees Fahrenheit. Place parchment paper in bottom of 8-inch square baking dish.
2. Whisk together flour, sugar, baking soda, and salt. In second bowl, combine cocoa, chocolate, espresso powder, and water; whisk until smooth. Into chocolate mixture whisk mayonnaise, egg, and vanilla until completely smooth. Add chocolate mixture to flour mixture and stir until combined.
3. Scrape batter into pan and smooth the top. Bake until cake surface is cracked and top has puffed. A wooden skewer inserted into center of cake comes out with few crumbs attached, 30 - 35 minutes.
4. Let cake cool in pan on wire rack, 1 to 2 hours. Run thin knife around pan edge to loosen cake. Invert and place on serving platter. Can be dusted with confectioner's sugar.
With thanks to Keith Dresser for his article in Cook's Illustrated, March 2009.