Mix the potato starch, sugar and lemon zest in a saucepan.
Gradually strain the lemon juice into the saucepan.
Add the water.
Heat over a medium heat, stirring constantly, until the consistency is smooth and thick.
When air bubbles appear, take the saucepan off the heat.
Now gradually add the butter.
Beat the yolks and whole eggs in a separate dish.
Add these to the lemon mixture and bring to a medium heat.
Keep stirring well until air bubbles form again.
Take the lemon curd off the heat.
Place in a piping bag and leave to cool.