1. Preheat oven to 350 degrees Fahrenheit.
2. Mix margarine, sugars, vanilla, and egg yolk. Stir in flour and salt.
3. With floured hands, press dough into bottom of ungreased nine- x 13-inch pan.
4. Beat egg white and water in a small bowl; brush over dough. Sprinkle with nuts of choice, pressing lightly.
5. Bake until light brown, about 25 minutes. Cut into three-inch squares, then cut each square diagonally in half. Immediately remove from pan. Cool completely.
6. Drizzle with chocolate. Let stand until chocolate is set, about two hours.