1.Preheat oven to 160°C (140°C fan-forced). Grease oven trays; line with baking paper.
2.Make peanut brittle. Place nuts on baking-paper-lined oven tray. Stir sugar and the water in small frying pan over heat, without boiling, until sugar is dissolved. Bring to the boil, boil, uncovered, without stirring, until golden brown. Pour mixture over nuts, leave until set. Crush coarsely in food processor.
3.Beat butter, peanut butter, sugar and egg in small bowl with electric mixer until combined. Stir in sifted dry ingredients and crushed peanut brittle.
4.Roll heaped teaspoons of mixture into balls with floured hands. Place about 5cm apart on oven trays, flatten slightly with hand.
5.Bake cookies about 12 minutes, cool on trays.