Make at least one day before serving.
8-inch round cake pan, 1 1/2–2 inches deep (not springform)
Baking pan at least 2 inches deep and large enough to hold the cake pan
1. Position the rack in the lower third of the oven and preheat to 350°F. Grease the sides of the pan and line the bottom with a round of parchment paper. Put a kettle of water on to boil.
2. Place the dark and milk chocolates in a large mixing bowl and set aside.
3. Combine the cocoa powder, flour, salt, and half of the sugar in a heavy 1- to 1 1/2-quart saucepan. Whisk in enough of the coffee to form a smooth paste. Mix in the remaining coffee. Cook over medium heat, stirring constantly to prevent burning (especially around the bottom edges of the pot), until mixture begins to simmer. Stirring constantly, simmer very gently for 2 minutes. Immediately scrape the hot mixture over the bowl of chopped chocolate. Stir until chocolate is completely melted and smooth. Whisk in the vanilla extract.
4. Beat the eggs and egg white with the remaining sugar at high speed until the volume doubles, about 5–6 minutes. The eggs will be very light and foamy. One-third at a time, fold the eggs into the chocolate mixture. Pour the resulting batter into the cake pan. Set the cake pan in the baking pan. Pour enough boiling water into the baking pan to come halfway up the sides of the cake pan. Bake until the cake has risen slightly and the surface springs back when very gently pressed, about 30 minutes. The cake will still jiggle in the center like very firm Jell-O, and the interior will be still quite gooey. Remove the cake pan from water and cool completely on a rack. Cover with plastic wrap and refrigerate overnight.
5. To unmold, slide a thin knife around the edges to release the cake from the pan. Cover with waxed paper and a plate. Invert, peel off the parchment liner, and invert again onto a serving platter. Refrigerate up to 2 days or freeze up to 2 months.
6. At serving time, sieve a light dusting of powdered sugar over the top, if desired. Serve unaccompanied, or with berries and little dollop of whipped cream if desired.
Calories: 198 calories, Carbohydrates: 32 g, Protein: 4 g, Fat: 8 g, Saturated Fat: 4 g, Cholesterol: 54 mg, Sodium: 48 mg, Fiber: 3 g
Exchanges per serving: 2 carbohydrate, 1 1/2 fat. Carbohydrate choices: 2.