Mix butter or margarine and caster sugar together until they are light and fluffy. Mix in some flour and knead the dough together until it forms a ball. Add a sprinkle of flour if the dough is too sticky.
Roll out the dough on a lightly floured surface until it is about 5mm thick.
Cut the dough into ‘traffic light’ rectangular strips. Place red, yellow and green smarties in a row down the strip.
Place the biscuits on a floured or lined baking tray and bake in the centre of a pre-heat oven at 150°C/300°F /Gas Mark 2 oven for 10 minutes or until golden brown. Let the biscuits cool on a wire tray.