Soak the gelatine leaves in cold water to soften.
Place 200ml of double cream and the caster sugar in a pan. Grate in a quarter of the tonka bean and then bring to the boil. Add the softened gelatine and the rest of the cream to the pan. Stir gently until the gelatine is completely dissolved.
Pour the mixture into individual moulds and chill for at least 45 minutes.
Add the instant coffee and the sugar to the espresso coffee.
Allow this liquid to cool to room temperature and then add the double cream and pour into a siphon. Attach the gas canister to the siphon and then chill in the fridge for 10 minutes.
Top the panna cottas with the coffee emulsion to serve.
«Try flavouring your panna cotta with different spices such as cinnamon or vanilla.»