Preheat oven to 275°F.

In a large mixing bowl, cream together butter and sugar until a fluff y smooth emulsion is formed, then add egg yolk. Once incorporated, add vanilla and flour while the mixer is going at the lowest possible speed. Dough will be very stiff .

Remove from bowl and place onto a jellyroll pan prepared with parchment paper. Use your clean hands to spread dough flat and evenly on a baking sheet, about ¼–½ inch thick.

Lightly beat the egg white with a hand whisk for about 30 seconds and brush over cookie dough. Sprinkle the chopped nuts over the top and gently press in.

Bake for 1 hour. Cool until just warm. Using a serrated knife or an ulu blade, cut long 1-inchwide slices in one direction at an angle, then again in the opposite direction, creating 1-inch diamond pieces.

Store in airtight container.