To make the biscuit base, crush the digestive biscuits with a rolling pin or blitz in a food processor, then tip into a bowl with the sugar and butter and stir to combine. Butter and line an 18cm springform tin and tip in the biscuit mixture, pushing it down with the back of a spoon. Chill in the fridge for 30 mins.
To make the cheesecake, melt the chocolate in short bursts in the microwave, then leave to cool slightly. Whip the cream in a large bowl using an electric whisk until soft peaks form, then fold in the cocoa powder. Beat the cream cheese and sugar together, then fold in the cream mixture and the cooled chocolate.
Spoon the cheesecake mixture over the biscuit base, levelling it out with the back of a spoon. Transfer to the freezer and freeze for 2 hrs, or until set. Remove from the tin and leave at room temperature to soften for about 20 mins before serving.