Place a lightly greased sheet of baking paper onto a baking sheet.
Heat the caster sugar in a saucepan over a medium heat without stirring, until the sugar dissolves and liquid turns a deep brown caramel colour.
Add the pistachios and pour the mixture onto the prepared baking sheet.
Allow the caramel to cool and set hard, then break it up into shards. Store the pieces between non-stick baking paper in an airtight container.