Spread the pistachios in a layer on a silicone mat on a baking tray.
Heat a clean, dry heavy-based saucepan over high heat for 1 minute or until hot, then add the caster sugar and cook without stirring for 8–10 minutes or until melted and a dry caramel forms. Immediately pour the hot caramel over the pistachios to coat. Set aside to cool and harden.