In a large bowl, combine the flour, salt, and softened butter. Rub the mixture between your fingers until it resembles fine crumbs and the butter is fully incorporated.
Add the caster sugar, yeast, and lukewarm water. Mix until a smooth, soft dough forms. Cover and refrigerate for 30 minutes.
Place the toasted sesame seeds in a shallow bowl. Gradually add 1–2 tablespoons of the cooled sugar syrup at a time, rubbing the sesame seeds between your hands until they become fragrant and lightly sticky. The seeds should cling together slightly but not form clumps.
Preheat the oven to 175°C (345°F) and line a baking tray with parchment paper.
Divide the rested dough into small, bite-sized pieces and roll each into a ball. Flatten gently.
If using pistachios, press one side of each piece into the chopped pistachios first, then press into the sesame mixture. Otherwise, coat directly in the sesame mixture.
Place the cookies on the prepared baking tray and flatten slightly.
Bake for 12–15 minutes, or until lightly golden. The cookies should remain pale and crisp rather than deeply browned.
Remove from the oven and allow to cool completely before serving.