A beautiful dessert for the waning summer and easily transportable to parties or picnics.
To make the fruit purée, put the redcurrants in a saucepan over a low heat and cook gently until the juice starts to run. Bring to the boil, stir well, then remove from the heat and rub through a sieve to extract as much pulp as possible. Discard the seeds. Stir the sugar into the purée until dissolved.
Put the peaches in a bowl of boiling water for up to 1 minute (ripe peaches will take less time). Remove and cool under cold water. Use the tip of a knife to loosen the skin, then peel it off. Cut the peaches in half and remove the stones. Cut the flesh into small slices and mix with the redcurrant purée. Divide this mixture among 4 glasses and chill.
Put the cream and the vanilla pod in a heavy-based saucepan over a low heat. Heat very gently until the cream comes to boiling point – about 15 minutes, if possible. (If it reaches boiling point quickly, turn off the heat and leave it to infuse for the remainder of the time.) Remove the vanilla pod.
Put the buttermilk in a heatproof bowl. Add the sugar to the cream and stir well to dissolve. Heat a little once more if necessary, to ensure the mixture boils. Immediately remove the pan from the heat and pour the cream mixture into the bowl containing the buttermilk. Stir well, then divide this posset mixture among the glasses. Chill for several hours. To transport to your picnic spot, put the glasses over a chilled ice pack in a box or other container so they remain upright.