Heat oven to 350ºF. Lightly grease a 15 1/2″ x 10 1/2″ x 1″ (half sheet) pan.
In the bowl of a standing mixer, combine butter, granulated sugar, and egg yolk.
In a separate bowl, sift together flour and cinnamon, and add to the butter mixture on low speed until thoroughly combined (mixture will be crumbly but will come together when pressed into pan).
Press dough into baking pan. Place plastic wrap or wax paper over the dough and use a small rolling pin to roll out dough evenly in pan.
Beat egg white and water until frothy and brush a light layer over dough, then sprinkle with nuts and sanding sugar.
Bake 20-25 minutes until lightly browned. Let rest in pan for 5 minutes.
Using a sharp knife, immediately cut into finger-like strips. Leave to cool and crisp up, before transferring to a wire cooling rack to cool completely.
