You’ll need 6-8 dariole moulds or small individual pudding basins. Tip 150g of the sugar into a shallow pan and place over a medium heat. Cook the sugar until dissolved and turning to a light amber caramel. While it is still hot, carefully pour the caramel over the base of the moulds, tipping them around so some caramel also sticks to the sides. Set aside.
Soak the gelatine in very cold water for about 10 mins to soften. Tip the cream, remaining sugar and the vanilla pod and seeds into a saucepan. Place on the heat, stirring to combine and dissolve the sugar. Bring to a simmer, stirring occasionally, then remove from the heat. Add the drained, squeezed gelatine leaves and stir to dissolve, then leave to cool. When tepid, remove and squeeze out the vanilla pod, stir in the buttermilk, then strain everything into a jug. Share the mixture between the moulds, then place in the fridge for at least a couple of hours, or preferably overnight to set. Can be made up to three days ahead.
To serve, dip the bases of the moulds briefly into very hot water until the fillings just come away from the sides. When you are confident that it will turn out, place a serving plate over the mould and carefully turn, allowing the contents to drop out onto the plate, scooping out any caramel sauce left in the mould and drizzling over the puddings. Serve immediately.