Slowly melt the butter in a small saucepan. Whilst the butter is melting crush the biscuits in a freezer bag. You can optionally use different utensils (e.g. melt the butter in a bowl in the microwave, crush the biscuits in a ziplock bag). Take the butter off the heat and mix in the crushed biscuit and sugar.
Line your cheesecake base (I used a cake tin) with parchment paper to prevent the cake from sticking. Pour on your butter and biscuit mix. Press and mould the mix into your desired base. Put in the fridge to chill for minimum 20 minutes.
Zest and juice 1 lemon.
Mix together the mascarpone cheese, caster sugar, lemon zest and lemon juice.
Once the base has chilled, pour on top the lemon cheesecake mixture. I recommend pouring it all into the middle of a base and, using a tablespoon, push out the mixture to the edge.
Chill in the fridge for 2 hours.