Preheat your oven to 175°C (350°F).
When your oven is heated, weigh our 65g of walnuts and toast for 5-8 minutes. Keep an eye on them so they don't burn, you'll know they're ready when your kitchen starts to smell toasty and fragrant.
Let your nuts cool a bit before chopping them finely or pulsing in a food processor. Set aside.
Add the butter and both sugars to a medium mixing bowl. Cream together until light and fluffy.
Crack your egg into a bowl and beat. Add about ¾ of your egg to your butter and sugar, mixing until well combined. You'll be reserving about 2 tablespoons of it for the egg wash later.
Next, add your cake flour, salt, baking soda, baking powder, and chopped walnuts. Mix until a smooth dough forms. The dough will be quite dry but keep working with it until everything is incorporated.
Using your hands form round balls about 37g each. You should end up with about 15 total.
Place your balls on a parchment or silicone lined baking sheet. They won't spread much, so just make sure they're evenly spaced. Press them a little bit with your palm to flatten. It's okay if cracks appear, that's all part of the look!
Take your 15 walnut halves and press them onto the top of each cookie.
Add a tablespoon of water to your reserved egg to make an egg wash. Gently brush each cookie with it.
Bake for about 20-23 minutes, until lightly golden brown on top.
Remove from the oven and let rest on your baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Eat straight away or store cookies in an airtight container for up to a week (but they won't be quite as crunchy by the end of it)!