1. Preheat oven to 350°.
2. To prepare cake, weigh or lightly spoon flour into a dry measuring cup; level with a knife. Sift together flour and 3/4 cup superfine sugar.
3. Place egg whites in a large bowl; beat with a mixer at medium-low speed until foamy (about 1 1/2 minutes). Add cream of tartar and salt; beat at medium speed until soft peaks form (about 2 minutes). Increase mixer speed to medium-high; add remaining 3/4 cup superfine sugar, 1 tablespoon at a time, beating until medium peaks form (about 2 minutes). Do not overbeat. Add 1 1/2 teaspoons vanilla, and beat just until combined.
4. Sift 1/4 cup flour mixture over egg white mixture; gently and gradually fold in. Repeat procedure with remaining flour mixture, 1/4 cup at a time.
5. Spoon batter into an ungreased 10-inch tube pan, spreading evenly. Bake at 350° for 40 minutes or until cake springs back when lightly touched. Invert pan over the neck of a glass bottle; cool cake completely. Loosen cake from sides of pan using a narrow metal spatula. Invert cake onto plate; invert again.
6. To prepare sauce, pour milk and half-and-half into a small heavy saucepan; add 2 tablespoons granulated sugar, dash of salt, and chocolate. Heat milk mixture over medium heat to 180° or until tiny bubbles form around edge, stirring until chocolate melts and mixture is smooth (do not boil). Place remaining 2 tablespoons granulated sugar, cornstarch, and egg yolk in a medium bowl; stir well with a whisk. Gradually add hot milk mixture to egg mixture, stirring constantly with a whisk. Return milk mixture to pan. Cook over medium-low heat until mixture boils, stirring constantly. Remove from heat. Stir in 1/4 teaspoon vanilla. Serve cake with warm sauce.