Step 1Heat the oven to 350 degrees. In the bowl of a stand mixer, or in a large bowl using a hand mixer, cream together the butter and sugar. Beat in the egg yolk until incorporated. Beat in the cinnamon, then slowly beat in the flour until incorporated.
Step 2Spread the dough evenly over the bottom of a lightly greased jelly roll pan (15 1/2 inches by 10 1/2 inches by 1 inch). Set aside.
Step 3In a small bowl, beat the egg white with the water until frothy. Brush the egg white mixture over the dough, then sprinkle over the chopped walnuts.
Step 4Bake until puffed and lightly golden, about 20 to 25 minutes. The cookies will settle and deflate after baking.
Step 5Cut the cookies while warm into small bars, approximately 2 inches by 1 inch. Store the cookies in a tightly wrapped container up to 2 days, or freeze (tightly covered) for up to 2 months.