Heat the cream, whole milk, sugar and Banana Shake pyramids (with the paper tags removed) in a saucepan.
Bring to a boil, stirring occasionally until all the sugar has dissolved, then reduce the heat and simmer until the volume of the liquid has reduced by half. Carefully squeeze the pyramid bag against the side of the pan to help release the flavour.
Take the pan off the heat and remove the Banana Shake pyramid, giving them a final squeeze.
Drain the gelatine leaves, add to the pan and stir until dissolved.
Pour into four ramekins or mini pudding bowls (these, from John Lewis, work perfectly). Set aside in the fridge for at least two hours until set.
To serve, dip the panna cotta into a bowl of hot water briefly or slide a flexible knife around the outside edge carefully. Turn the panna cotta out onto a plate. Serve with shards of dark chocolate for an extra treat