Photograph: Tara Fisher
Soak the gelatine in cold water. Heat the cream with the sugar and a good teaspoon of zest from the orange in a pan over a low heat, stirring until the sugar dissolves. Remove from the heat. Squeeze out the excess water from the gelatine leaves and stir them into the hot cream mixture until dissolved. Stir in the buttermilk and vanilla. Divide between 6 x 175ml pudding basins or ramekins, cover with clingfilm and refrigerate until set – at least 3 hours.
Preheat the oven to 180°C, fan 160°C, gas 4. Fold a large piece of kitchen foil to make a double-thickness square measuring about 30cm. Turn up the edges slightly and put the plums in the middle. Top evenly with the remaining ingredients and pull up the sides of the foil to enclose the plums and flavourings, making sure no juices will leak out. Bake for 15-18 minutes or until the plums are tender. Leave to cool until barely warm.
Dip the base of the pudding basins or ramekins in boiling water for a few seconds, one at a time, so the panna cotta will slide out easily. Turn on to serving plates, garnish with the remaining zest and serve with the plums and juices – I sometimes tip these into a pan and bubble briefly to make them more syrupy.
Get ahead: make the panna cottas up to 2 days ahead; cover and chill. Assemble the plum parcel a few hours ahead.
Kitchen secret: buttermilk is a light dairy product. It’s a by-product of making butter – the butter takes all the fat and the buttermilk left is light and slightly tangy. It goes beautifully with plums. If you like, you could make one large panna cotta in a shallow serving dish.