Preheat the oven to 300 degrees F (150 degrees C). Set out two rimmed baking sheets.

Beat butter and sugar with an electric mixer in a mixing bowl until light and fluffy. Mix in egg yolk until thoroughly combined and creamy. Add flour and cinnamon and mix until dough is well combined.

Spread 2/3 of the dough over one baking sheet; spread remaining dough over half of the second baking sheet. Press down on dough with your fingers, then score with a fork.

Whisk egg white and water together in a small bowl; brush lightly over dough. Sprinkle walnuts over top.

Bake in the preheated oven for 30 minutes.

Remove from the oven and immediately cut into 24 squares or rectangles. Let cool completely on the baking sheets, then remove and separate cookies.