Heat oven to 300º F. Using an electric mixer, beat the butter, brown sugar, maple syrup, and egg yolk until smooth. Stir in the flour, cinnamon, and salt until combined. Spread the dough into an ungreased 15 1/2-by-10 1/2-inch jelly-roll pan or cookie sheet with sides.
Beat the egg white in a small bowl with a fork until foamy. Spread evenly over the dough. Sprinkle on the nuts and press them gently into the dough.
Bake 35 to 40 minutes or until the shortbread shrinks slightly from the sides of the pan. Do not overcook; you want them to be a little chewy. Cut diagonally into 3-by-1 1/2-inch bars.