Preheat the oven to 180ºC/350ºF/gas 4, and grease and line the bottom and sides of a 24cm springform cake tin.
Melt the butter, and crush the biscuits, then mix together in a bowl. Lightly press into the base of the prepared tin and bake for 10 minutes. Remove from the oven and leave to cool.
Turn the oven up to 200ºC/400ºF/gas 6.
Combine the sugar and cornflour in a bowl, add the cream cheese and beat, ideally with an electric whisk, until creamy.
Crack in the eggs and beat well, then gradually add the cream, beating until smooth.
Halve the vanilla pod lengthways, then scrape out the seeds and add to the bowl (if using), or add the vanilla extract, then finely grate in the lemon and orange zest.
Scrape the mixture on to the biscuit base, and gently shake to level out the surface.
Bake the cheesecake in the centre of the oven for 40 to 45 minutes, or until the top is golden and the filling has set around the edges (cover with a piece of tin foil, if browning too much).
Let it cool at room temperature and serve after 2 or 3 hours, or, for a slightly firmer texture, place it in the fridge until it’s nice and cold.
Once reduced, it may be a little dry, so add a splash of water to loosen, if needed. Leave to cool, then serve spooned over the cheesecake with a dusting of icing sugar.