Slit the vanilla pod lengthways and scrape out the black pulp with a knife. Heat the pulp and pod together with cream, sugar and salt in a saucepan over low heat while stirring until the sugar has melted. Do not let it boil!
Take the saucepan off the stove and let the mixture cool down again, then stir in the double cream and remove the vanilla pod.
Put the cream mixture in a freezer-proof bowl and cover it on the freezer shelf. Let the ice freeze for at least 4 hours.