Chill a large jug or bowl in the freezer for the panna cotta.
Heat double cream, vanilla, and sugar in a saucepan until just boiling.
Whisk in soaked gelatine and buttermilk off the heat.
Sieve the mixture into the chilled jug and stir in vodka.
Chill and whisk every five minutes until setting begins.
Pour into moulds and refrigerate for at least two hours.
Preheat oven to 180°C and line an 18cm cake tin.
Simmer sugar, dripping, dried fruit, peel, and 225ml water for 10 minutes.
Mix dry ingredients and stir in the cooled fruit mixture to form a batter.
Bake in the tin for 40 minutes or until a skewer comes out clean.
Cool the cake in the tin for five minutes then transfer to a rack.
Preheat oven to 200°C for the rhubarb.
Roast rhubarb with sugar and apple juice for 8 to 12 minutes.
Purée 125g of the roasted rhubarb in a food processor.
Unmould panna cottas and serve with purée, roasted rhubarb, and cake slices.