First, prepare your raspberries. Put the sugar, cassis and 50ml of water in a pan over a low heat and stir until the sugar has dissolved. Pour it over the raspberries and leave to macerate for at least 2 hours.
For the shortbread, chop the butter into cubes, scrape the seeds from the vanilla pod (discard the pod) and zest the lemon, then mix all the ingredients except the granulated sugar in a bowl, until it forms a dough, but be careful not to overwork it. Mould it into a block, wrap in clingfilm and chill in the fridge for 1 hour.
Preheat the oven to 160ºC/gas 2-3.
On a lightly floured surface, roll the dough to 1cm thickness. Bake for 20 to 25 minutes, or until lightly golden all over.
Remove from the oven and cut with a star cutter, then sprinkle with the granulated sugar (use the biscuit off-cuts for nibbling with a cup of tea!)
To make the posset, finely grate the zest from one of the lemons and one of the oranges. Put the cream, sugar and lemon and orange zests in a saucepan over a medium-high heat.
Bring to the boil and cook for 3 minutes. Add all the citrus juice, bring back to the boil and continue to cook for another 3 minutes.
Pass the mixture through a fine sieve and pour it straight into six champagne saucers or wine glasses while still hot (but not boiling, or the glass might crack). Allow it to set for at least 3 hours, or overnight.
To serve, remove the glasses from the fridge and garnish each one with
a few raspberries, baby mint leaves (if using), a shortbread star and strips of candied peed (if using).