For the base: Melt the butter in a large pot over medium heat, then add the marshmallows and stir until starting to melt. Add the melted white chocolate and stir until completely melted. Remove the pot from the heat.
Add the popcorn to the pot and stir to coat completely. Let cool for 2 minutes.
Grease a 23-cm springform pan with nonstick spray. Press the popcorn mixture into the pan using the bottom of a measuring cup, creating an even layer along the bottom and halfway up the sides. Set aside.
To make the sauce: Heat the butter, cream, sugars and salt together in a medium non-stick pan, and, stirring continuously, bring to the boil. Turn down to a simmer and cook for 10 minutes, stirring occasionally. Remove from the heat and set to one side. If, when it’s cooler, the mixture is stiff, warm it up over the heat with a splash of milk to loosen it.
To make the cheesecake: Put the cream cheese, mascarpone, icing sugar and vanilla into a mixing bowl or the bowl of a stand mixer, and beat it together. Mix until well combined and thickening, then add the cream and continue to mix. Pour in the melted white chocolate and whisk to combine then finally pour in about 150 ml of the butterscotch sauce and fold it through the cream cheese mixture to combine. Pour into the tin and smooth the top with the back of a spoon or spatula. Leave to set in the fridge for at least 6 hours.
When ready to serve, remove the cheesecake from the tin, scatter with the caramel popcorn and drizzle with the remaining butterscotch sauce.