1. Preheat the oven to 375 degrees.
2. Cream the butter and sugar together very well. Beat in the egg yolk, vanilla, and salt.
3. Stir in the whole wheat flour and unbleached white flour. Press the mixture into a well-buttered 15- by 10- by 1-inch jelly roll pan. Sprinkle with the nuts and press them in. Beat the egg white lightly and brush over the top.
4. Bake for 20 minutes or until well browned and done. Cut into bars or squares while hot. Cool in the pan.