Beat the butter and icing sugar together until smooth.
Push the hard-boiled egg yolk through a sieve and stir the raw egg yolk and vanilla into it. Add this to the butter mixture and stir until blended.
Add the flour and salt to the butter mixture and stir until blended. Shape the dough into a disc (it will be very soft), wrap in plastic and chill until firm, about 2 hours.
The dough can be prepared and frozen for up to 3 months. Thaw in the fridge before using.