From Cook's Illustrated.
Ingredients
2 teaspoons instant coffee
1 1/2 cups water
1/3 PLUS 1/3 cup Dutch-processed cocoa powder (2 1/2 ounces total)
1/3 cup brown sugar (packed, 1 3/4 ounces)
1/3 PLUS 2/3 cup granulated sugar (7 ounces)
6 tablespoons unsalted butter
2 ounces semisweet chocolate or bittersweet chocolate, chopped
3/4 cup unbleached all-purpose flour (3 3/4 ounces)
2 teaspoons baking powder
1 tablespoon vanilla extract
1/3 cup whole milk
1/4 teaspoon table salt
1 large egg yolk
BEFORE YOU BEGIN
If you have cold, brewed coffee on hand, it can be used in place of the instant coffee and water, but to make sure it isn’t too strong, use 1 cup of cold coffee mixed with 1/2 cup of water. Serve the cake warm with vanilla or coffee ice cream. Leftovers can be reheated, covered with plastic wrap, in a microwave oven.
Instructions
1. Adjust oven rack to lower-middle position and heat oven to 325 degrees. Lightly spray 8-inch square glass or ceramic baking dish with nonstick cooking spray. Stir instant coffee into water; set aside to dissolve. Stir together 1/3 cup cocoa, brown sugar, and 1/3 cup granulated sugar in small bowl, breaking up large clumps with fingers; set aside. Melt butter, remaining 1/3 cup cocoa, and chocolate in small bowl set over saucepan of barely simmering water; whisk until smooth and set aside to cool slightly. Whisk flour and baking powder in small bowl to combine; set aside. Whisk remaining 2/3 cup sugar, vanilla, milk, and salt in medium bowl until combined; whisk in yolk. Add chocolate mixture and whisk to combine. Add flour mixture and whisk until batter is evenly moistened.
2. Pour batter into prepared baking dish and spread evenly to sides and corners. Sprinkle cocoa/sugar mixture evenly over batter (cocoa mixture should cover entire surface of batter); pour coffee mixture gently over cocoa mixture. Bake until cake is puffed and bubbling and just beginning to pull away from sides of baking dish, about 45 minutes. (Do not overbake.) Cool cake in dish on wire rack about 25 minutes before serving.