Preheat the oven to 180'C.
Dice the butter. Split the vanilla pods in half and scrape out the seeds.
Place all of the ingredients in a food processor and pulse until a ball begins to form. Do not overwork in the food processor. Remove and finish kneading by hand until you have a firm ball of dough. Roll the pastry between 2 sheets of baking paper until 5mm thick. Bake in the oven for 10-12 minutes. Remove and cut into shapes while the shortbread is still soft. Return to the oven and bake for a further 3 minutes or until golden brown on the edges and firm in the middle. Sprinkle with the caster sugar whilst still hot and then allow to cool.
Place the gelatine leaves in a bowl of cold water to soften. Zest and juice the limes. Bruise the lemongrass with the back of a knife.
Place the milk, cream and the sugar in a saucepan over a medium heat. As this mixture comes up to the boil, add the lemongrass. Bring to the boil and then remove from the heat and allow to infuse for a couple of minutes. Add the softened gelatine leaves to the mixture and stir to dissolve. Pass the mixture through a fine sieve into a bowl, mix in the lime zest and half the juice, then pour into individual moulds. Refrigerate for at least 1 hour.
Immerse the moulds into warm water to turn the out and then spoon over a little of the remaining lime juice over.
Serve with the ginger shortbread.
«If you need to chill your panna cotta really quickly, sit the moulds in a tray filled with ice and refrigerate.»