Begin preparing about 2 hours before serving: In a 2-qt. saucepan, combine flour and 1/4 cup sugar; slowly whisk in milk until smooth. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil for 1 minute. Remove from the heat; stir in chocolate until melted. Rapidly beat in egg yolks, all at once, until well mixed. Refrigerate until lukewarm, stirring occasionally. In a large bowl, beat egg whites and salt until soft peaks form. Gradually add remaining sugar; beat until dissolved and stiff peaks form. Gently fold in chocolate mixture, one-third at a time, and vanilla until blended. Pour into a buttered and sugared 2-1/2-qt. souffle dish or round baking dish. With the back of a spoon, run a groove about 1-1/2-in. deep all around top, about 1-1/4-in. from edge of dish.
Bake at 375° for 35-40 minutes or until a knife inserted under "top hat" comes out clean. Remove from the oven; dust with confectioners' sugar and serve immediately.