Before you start, make sure you have space for a flat tray in your freezer. arrange half the wafer biscuits on the tray. Spoon the ice cream into a food processor and pulse until just smooth (be careful not to not let it melt too much). Spoon quickly into a disposable piping bag, cut off the end (a little narrower than the width of a biscuit) and pipe an ice cream log onto the biscuits. Try to squeeze from the end of the bag, so that the heat from your hands doesn’t melt the ice cream.
Place a second biscuit on top to complete the sandwiches and press together a little to squeeze the ice cream to the edges. Put the tray into the freezer and freeze for at least an 1 hr.
Melt the chocolate in a microwave or in a heatproof bowl over a pan of simmering water. Tip the pistachios into a dish. Working quickly, dip the end of each frozen sandwich into the melted chocolate, then into the pistachios. Put back onto your tray and freeze again for 1 hr. Will keep for 1 month frozen in an airtight container between sheets of baking parchment.