Place heavy cream, milk and caster sugar in a pot and bring to a simmer just until the sugar melts. Don't let it boil! Remove from the stove,
Meanwhile, soak gelatine leaves in cold water as instructed on the label.
Cut white chocolate into small chunks or grate and add to the pot. Mix well until chocolate has melted. Add gelatine and mix again until the gelatine has dissolved.
Pour into jars or molds and immediately place in the fridge to cool down. Leave to set for 2-4 hours before serving.
Meanwhile, prepare the sauce. Steep hibiscus tea in boiling water for ten minutes. Discard the tea bags and add caster sugar to the tea. Cook tea over medium heat until it reduces to half it's volume.
Add raspberry purée and mix well. The sauce will thicken when it cools.
Pour sauce over panna cotta before serving.