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.
2. 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.
3. 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 large bowl until combined; whisk in yolk. Add chocolate mixture and whisk to combine. Add flour mixture and whisk until batter is evenly moistened.
4. 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.
INDIVIDUAL HOT FUDGE PUDDING CAKES
Follow recipe for Hot Fudge Pudding Cake, heating oven to 400 degrees and lightly spraying eight 6- to 8-ounce ramekins with nonstick cooking spray; set ramekins on baking sheet. Divide batter evenly among ramekins (about 1/4 cup per ramekin) and level with back of spoon; sprinkle about 2 tablespoons cocoa/sugar mixture over batter in each ramekin. Pour 3 tablespoons coffee mixture over cocoa/sugar mixture in each ramekin. Bake until puffed and bubbling, about 20 minutes. (Do not overbake.) Cool ramekins about 15 minutes before serving (cakes will fall).
**FROM America's Test Kitchen: Public television's most watched cooking show.