Sift the flour once; then add baking powder and salt, and sift together 3 times. Cream the butter and sugar until light and airy. Add all the eggs, and continue to beat. Add flour alternately with the milk, sm. Amts. At a time, beating each time, until smooth. Add the flavoring. Bake in 3 buttered layer pans in a 375 oven for 20 minutes. For Filling: Combine sugar, butter, vanilla, salt, chocolate, and milk in a deep bowl, and beat with a rotary egg beater until well blended. Place bowl in a pan of ice water and continue to beat until it thickens to spreading consistency. This goes between the layers. Frosting: Combine egg whites, sugar, and water with the corn syrup in top of double boiler; and beat with egg beater til thoroughly mixed. Place over boiling water, and beat constantly for 7 minutes, or till frosting stands in peaks. Remove, and add vanilla; beat until thick enough to spread on top and sides of cake.