For the base, crush the biscuits and mix them with the melted butter. Press into the bottom of a 20cm spring-form tin. Chill.
For the filling, mix the sugar, cream cheese and seeds from the vanilla pod together. Whisk the lemon juice with the cream and creme frache to soft peaks. Fold everything together. Keep the mixture cool until you need it, but don’t chill it.
For the swirl, melt the cream and chocolate together in a bowl set over a pan of simmering water. Cool the mixture until it is cold enough not to melt the filling and thick enough to hold a trail.
Spoon or pipe a layer of cheesecake mix over the biscuit base, then drizzle a thin stream of chocolate mix in a swirl on top, then repeat until you have three layers of each. Stick a thin knife blade or skewer into the cheesecake and swirl the mixtures together. Don’t be tempted to smooth the surface down or it will look muddy. Chill for at least 5 hours and keep chilled until you need to cut it.