1. Heat oven to 350 degrees F.Prepare cake:
2. Line bottom of 2 straight-sided 8-inch-round baking pans with waxed paper. Sift flour, baking powder, salt onto sheet of waxed paper.
3. Beat whites in a bowl until soft peaks form. Beat in 1/2 cup sugar for 1 to 2 minutes until stiff peaks form.
4. Beat yolks and remaining 1/2 cup sugar in large bowl until thickened and light in color. Beat in water and vanilla. Fold in whites. Fold in flour mixture in 2 additions. Scrape into pans.
5. Bake in 350 degrees F oven 25 minutes or until tops spring back when lightly touched. Cool pans on racks for 20 minutes. Turn cakes out onto racks to cool completely. Peel off paper.Prepare Filling:
6. Mix sugar, flour and salt in saucepan. Stir in 3/4 cup milk. Heat over medium heat, whisking constantly, until mixture boils; cook 2 minutes. Remove from heat. Whisk together remaining milk and egg. Whisk mixture into saucepan. Return to heat. Cook, stirring, 1 minute. Stir in vanilla. Cover with plastic wrap, pressing on surface to prevent film. Cool.
7. Invert cake layer on serving plate. Stir cooled filling; spread over layer. Invert second layer on top.Prepare glaze:
8. Melt butter and chocolate in saucepan over low heat, stirring occasionally. Stir in corn syrup and water until blended. Stir in confectioners sugar until smooth. Spoon on top of cake; spread level, allowing some of glaze to flow over edge. Chill.