Directions

1. Position rack in lower third of oven; preheat to 350 degrees F. Line the bottom of an 8- or 9-inch cake pan (1 1/2 to 2 inches deep) with parchment paper and coat the sides of the pan with cooking spray. Put a kettle of water on to boil for Step 6.
2. Place chocolate and cocoa powder in a large bowl.
3. Combine flour, 2/3 cup sugar and salt in a small heavy saucepan. Whisk in just enough of the milk to form a smooth paste. Mix in the remaining milk. Cook over medium heat, stirring constantly with a wooden spoon to prevent burning (especially around the sides and bottom edges of the pot), until the mixture begins to bubble. Boil gently, stirring constantly, for 2 to 2 1/2 minutes (the mixture will get very thick and then you may notice that it thins ever so slightly as the starch cooks). Scrape the hot mixture immediately over the chocolate and cocoa. Stir until the chocolate is completely melted and smooth. The batter will be very thick. Stir in egg yolks and vanilla.
4. Beat egg whites and cream of tartar in a mixing bowl with an electric mixer on medium speed until soft peaks form. Gradually sprinkle in the remaining 1/4 cup sugar, beating on high speed until the egg whites are stiff but not dry.
5. Gently fold about one-fourth of the egg whites into the chocolate batter to lighten it. Gently fold in the remaining egg whites until no white streaks remain. Scrape the batter into the cake pan, smoothing the top.
6. Set the cake pan in a larger baking pan and place on the oven rack. Pour enough boiling water into the baking pan to come a third to halfway up the side of the cake pan. Bake until the surface of the cake is slightly crusted and springs back when gently pressed, 20 to 25 minutes. (The cake will still be quite gooey inside.)
7. Meanwhile, to prepare candied orange peel (if using): Using a vegetable peeler, remove zest from oranges in 1 1/2- to 2-inch-long pieces. Cut the pieces into very thin strips, about 1/8 inch wide. Cook in a small saucepan of boiling water for 5 minutes. Drain. Bring more water to a boil and cook the orange peel for another 5 minutes. Drain.
8. Bring sugar and 1/4 cup water to a simmer in a small saucepan, stirring until the sugar dissolves. Add the orange peel, cover and simmer for 3 minutes. Transfer the syrup and peel to a bowl. Cover and chill overnight.
9. Using a slotted spoon, transfer the orange peel to paper towels to drain before using.
10. Remove the pans from the oven. Transfer the cake pan to a wire rack and cool completely, about 2 hours. Cover with plastic wrap and refrigerate overnight before serving.
11. To serve: Soak the blade of a thin knife in a cup of very hot water until warm. Slide the knife around the sides of the pan to release the cake. Place a piece of wax paper on top of the cake. Invert a plate over the wax paper and invert the pan onto the plate. Remove the pan and peel away the paper liner. Place a serving plate over the cake and turn the cake right-side up again; remove the wax paper. Dip a sharp knife in hot water and wipe it dry before cutting each slice. Serve with Candied Orange Peel, if desired.