Preheat the oven to 125˚C (240˚F) Gas 1. To make the crust, crush the digestive biscuits until you get fine crumbs. Add the melted butter – the amount of butter you will need is variable.
Test by grabbing a bit of the mixture and squeezing into your hand to make a ball, then releasing your hand. The mixture should hold its shape, but also fall apart when touched slightly. If it doesn’t hold its shape, add more butter, otherwise the biscuit will dissolve into the cheesecake and you’ll have no crust. If it holds its shape too well, add more biscuits to absorb the butter, otherwise your crust will be too hard. Press the mixture into the prepared cake pan and pat down until level.
Put the cream cheese and sugar in a bowl and beat until well mixed and the sugar has dissolved. Slowly incorporate the eggs, one at a time, beating until thoroughly combined before adding the next. Scrape the side of the bowl regularly to make sure everything is incorporated.
Sift the cornflour into the mixture and stir until thoroughly combined. Add the cream and vanilla extract and mix until combined.
Pour the mixture into a lined 25cm cake pan over the crust and bake in the preheated oven for 1 hour 20 minutes until the middle is slightly jiggly and the top doesn’t look shiny or wet any more. Remove from the oven and let cool in the pan for 1 hour.
Refrigerate overnight. Unmould the cheesecake by turning it upside down on a plate or board, then uprighting again. To make the topping, put the cream in a bowl and, using an electric mixer with whisk attachment (or an electric whisk), whisk to stiff peaks. Fold in the mascarpone, icing sugar and vanilla extract. Spread the topping casually over the cheesecake and top with all the berries. Dust icing sugar over the top before serving.