Add Minute tapioca, sugar, salt to milk, cook in double boiler 15 minutes, or until tapioca is clear. Stir frequently. Pour small amount of mixture slowly over egg yolk. Stir vigorously. Return to double boiler. Cook until mixture thickens. Remove from heat, add butter and vanilla. When lukewarm, stir in berries sweetened with 2/3 cup sugar. Fold in egg white. Chill. Spread between layers and over top of cake. Serves 8.