1. Rub butter into flour, using fingertips, until mixture resembles fine breadcrumbs. Stir in sugar and carob. Add egg, stir with a knife until just combined, knead lightly until smooth.
2. Roll out to 5 mm / 1/4 in thick on a lightly floured surface. Cut into circles using a 5 cm / 2 in cutter. Place onto a lightly greased baking tray. Mark with prongs of a fork.
3. Bake in moderate oven 10 minutes or until golden brown, place on wire rack to cool. Store in airtight container.