Preheat the oven to 180C/350F/Gas 4/180C. Grease two baking trays.
Using a hand-held electric mixer, cream the butter until it is soft and fluffy. Gradually add the icing sugar, mixing on a low speed until it is all combined, then increase the speed and beat until light and creamy.
Gradually add the flour and custard powder, beating as you do so. The mixture should be loose enough to pipe through a piping bag; add a little cold water if needed.
Spoon in the biscuit mixture into a piping bag fitted with a large star nozzle, squeeze the mixture down to remove any air bubbles. Pipe 24 stars onto the baking trays, 4cm/1½in apart. Bake in the oven for 10-14 minutes, or until the biscuits are pale golden-brown at the edges.
Remove the trays from the oven and set aside to cool completely.
To make the custard cream filling, beat the butter using an electric hand mixer until light and fluffy, then add the icing sugar and beat at a low speed to combine. Add the custard powder, continue beating until thoroughly combined then set the mixture aside. Spoon the filling into a piping bag fitted with a plain nozzle.
To make the jam, blend the strawberries in a food processor or with a hand blender to make a purée. Place the purée in a saucepan and add the sugar. Heat gently until the sugar has dissolved then add the butter. Turn the heat up and when the mixture has reached a rolling boil continue boiling for four more minutes. Remove the pan from heat and pour the jam into a bowl to cool.
Spoon the jam into a piping bag fitted with a small plain nozzle.
Check the biscuits are completely cool then use a palette knife to lift them onto a clean tea towel, flat side uppermost, pairing them up as you go. Be careful as the biscuits are very fragile.
Pipe a circle of custard cream filling onto half the stars, then fill with the jam. Place the paired star on top, pressing it down gently.