Photograph: Dan Jones
Lightly grease 2 x 120ml dariole moulds with the vegetable oil (if you don't have any moulds, you can use little glasses instead).
Put the gelatine leaf in a shallow bowl and cover it with just enough cold water to submerge it; leave to soak for 5 minutes.
Split the ½ vanilla pod down the middle and scrape the seeds into a small pan. Pour in the cream and add the sugar. Keep the heat low until the sugar has dissolved; turn it up to bring the mixture to simmering point. Remove from the heat.
Squeeze the excess water out of the gelatine, add it to the cream mixture and stir until dissolved.
Stir in the milk and buttermilk; then pass the mixture through a fine sieve, discarding the vanilla pod.
Divide the mixture between the moulds, cool, cover with clingfilm and transfer to the fridge for at least 2 hours until set.
For the syrup, put the Amaretto and sugar into a small pan and gently heat to dissolve the sugar. Then turn the heat up and boil for 3-4 minutes to reduce the liqueur to a syrup that will lightly coat the back of a spoon. Remove from the heat and cool.
Take the panna cotta out of the fridge 15 minutes before serving. Remove the clingfilm; dip the bottom of the moulds into hot water before turning the panna cotta out on to plates. Spoon over the Amaretto syrup and crumble over the amaretti biscuits.