- Take the butter out of the fridge and leave until room temperature.
- Cream the softened butter and sugar together in a bowl. Once combined, add the egg and mix well.
- Sift the flour into the bowl, mix well, and form the dough into a ball.
- Wrap the dough with cling film and chill for half an hour in the fridge.
- Preheat your oven to 170C.
- Take the dough out of the fridge and roll out onto a floured surface to 2-3mm thick. Cut out fish shapes with biscuit cutters if possible, otherwise use a butter knife to gently cut out the shapes avoiding damage to the working surface. Make half of the fish-shaped biscuits larger (about 8cmx3.5cm) than the rest of the dough. With the other half, make small ones (about 6cmx3.5cm). You can change the sizes if you want.
- Place the shapes on a parchment-lined baking tray and bake for 12-15 minutes. Check the colour after 10 minutes.
- When golden brown, remove from the oven and cool on a wire rack.
- When cooled, decorate with colour icing pens or hand-made icing sugar to make them look like cute koi fish.
- A traditional koinobori banner has koi fish representing each member of the family. Larger ones represent the mother and father, while the smaller ones represent the children in your family.