Slit open the vanilla pod lengthwise and scrape out the seeds. Pour the cream and buttermilk into a saucepan.
Add the sugar, vanilla seeds and vanilla pod and simmer over a low heat for about eight minutes.
If you are using vanilla essence instead of the pod, add the essence at the end of the simmering time.
Remove from the heat and remove and discard the vanilla pod.
Put the gelatine into a microwave-safe jug, add 25ml cold water and leave to swell.
Heat in the microwave for a minute or two until the mixture has melted.
Stir until smooth, then stir into the cream mixture.
Rinse four small moulds or ramekins in cold water. Fill with the panna cotta mixture cover with clingwrap and refrigerate for five hours.
Whisk the egg white lightly. Pick up individual flowers using tweezers, dip briefly in the egg white and sprinkle with castor sugar.
Set aside to dry on a baking tray lined with baking paper.
Repeat to coat all the flowers, and allow to dry for about two hours.
To serve, remove the moulds from the fridge and use a sharp, non-serrated knife to loosen the edges of the desserts.
Hold a warm cltoh over the mould for a few seconds, then invert onto a serving plate.
Garnish with the sugared flowers and serve as is, or drizzle a berry coulis or berry sauce around the plate before serving.