Blitz the biscuits in a food processor to a fine crumb, or bash up in a bowl with a rolling pin, or in a food bag.
Melt your butter carefully so its liquid, and then mix in with the biscuits. I usually just pour it into the food processor and blitz again.
Press the Biscuit mixture into the bottom of an 8"/20cm springform deep tin, and refrigerate/freeze whilst you make the filling.
For the Cheesecake
In a large bowl, or stand mixer, add your cream cheese, icing sugar, vanilla and lemon juice and whisk till smooth. I either use my stand mixer with its whisk attachment, or a hand held electric whisk with their whisk attachments.
Whisk on a medium speed, until the sugar is mixed into the cream cheese, and the ingredients are evenly distributed. Usually takes about 15-25 seconds or so.
Add in the liquid double cream, and whisk the mixture till it comes together and becomes thick.
You can alternatively whisk up the cream separately until stiff peaks, and fold through the cheese mixture.
I usually mix it on a medium speed, and it should be really thick once done. Its best to under whisk compared to over whisking. If in doubt, whisk a bit, and then mix with a spatula to see how its doing.
Once whisked, add the cheesecake mixture to the top of the biscuit base, and set it in the fridge for 5-6 hours, or preferably overnight.
For the Decoration
Whisk together the double cream and icing sugar until pipeable, and then decorate your cheesecake.
Serve your cheesecake as is, or with a chocolate sauce, caramel sauce, berries, coulis... anything you fancy!
You can use either mascarpone, or Philadelphia cream cheese (or the supermarket version), but it MUST BE full fat cream cheese.
I'm based in the UK, so we have double cream. It has a rough fat content of around 45% and this is what helps it set. If you don't have access to it, use the nearest you can find.
If you want to make it lower fat you can, with lower fat cream cheese, but it won't set the same, and its more likely to split.