Line the base and sides of a 23cm round spring-form tin with parchment paper.
In a food processor, combine the biscuits and melted butter until the mixture is damp and coarse. Pour the crumb into the prepared tin and press firmly halfway up the sides and evenly into the base. Place in the refrigerator to set.
In a stand mixer bowl with the whisk attachment, combine the cream cheese, sour cream, caster sugar, and eggs. Whisk until smooth.
In a small bowl, mix the custard powder, S'mores vanilla extract, and water into a smooth paste. Add to the cream cheese mixture with the motor running, whisking to combine.
Place the chopped chocolate in a heatproof bowl over simmering water and melt, stirring until smooth. Slowly add the melted chocolate into the cream cheese mixture with the motor running, whisking until well combined.
Pour the filling over the chilled biscuit base.
Bake for 60 minutes, reducing the temperature to 150°C for the last 5 minutes. The sides will be set with a slight wobble in the middle.
Turn the oven off and leave the door slightly ajar to allow the cheesecake to cool.