Preheat fan forced oven to 160ºC, and prepare cookie sheets by lining with baking parchment.
To make the biscuits, cream the butter and icing sugar together using an electric mixer or beater. Once this mixture is pale and creamy, add the flour, custard poder and salt. Gently mix until just combined.
Transfer the dough to a piping bag fitted with a star nozzle, and pipe the desired size onto prepared baking trays. [These are rich, so I suggest keeping them small, becasue they will be sandwiched!]
Bake for 7-10 minutes or until just set. [They should not become golden, but their appearance should look dry, and you will be able to slide them on the paper with your fingers.]