Prepare this dessert at least one day before serving.
5 ounces bittersweet or semisweet chocolate, finely chopped
1 large egg 1 large egg, separated
1 teaspoon vanilla extract
1 large egg white
1/8 teaspoon cream of tartar
1/2 cup plus 1/2 tablespoon unsweetened Dutch processed cocoa
2 tablespoons all-purpose flour
2/3 cup plus 1/4 cup granulated sugar, divided use
3/4 cup low-fat milk Lightly sweetened whipped cream for accompaniment
Adjust oven rack to lower third and preheat to 350*F (175*C). Line 8-inch round cake pan that's 1 1/2 to 2-inches deep with parchment paper and spray with non-stick vegetable spray.
Place chocolate in a large bowl. In a separate small bowl, combine 1 whole egg and 1 yolk with vanilla. Place 2 egg whites in a large bowl or bowl of electric mixer with cream of tartar. Set all 3 bowls aside.
Combine cocoa, flour and 2/3 cup sugar in a 1 1/2-quart heavy bottomed saucepan. Whisk in enough milk to form a smooth paste. Mix in remaining milk. Cook over medium heat, stirring constantly with a wooden spoon, to prevent burning (especially around bottom edge of pot), until mixture begins to simmer. Simmer very gently, stirring constantly, for 1 1/2 minutes. Pour hot mixture immediately over chopped chocolate. Stir until chocolate is completely melted and smooth. Whisk in egg and vanilla mixture. Set aside.
Beat egg whites and cream of tartar at medium speed until soft peaks form. Gradually sprinkle in remaining 1/4 cup sugar, beating at high speed until stiff but not dry. Fold a quarter of egg whites into chocolate mixture to lighten it. Fold in remaining egg whites. Scrape mixture into cake pan and smooth the top. Set cake pan in baking pan and place on oven rack. Pour enough boiling water into baking pan to come 1/3 way up side of cake pan.
Bake 30 minutes. The surface of cake will spring back when gently pressed but will still be quite gooey inside. Remove cake pan and water pan from oven. Remove cake pan from water and cool completely on rack. Cover with plastic wrap and refrigerate overnight before serving. Refrigerate up to 2 days or freeze up to 2 months.
To serve, slide a knife around the sides of the pan to release cake. Invert cake on plate. Remove pan and peel away parchment paper. Cut with a sharp knife that has been dipped in hot water and dried. Serve with whipped cream.