Lightly grease and line a 20cm springform cake tin. To make the base, put the biscuits and butter in a food processor, pulse into fine crumbs, then tip into the tin. Spread evenly over the base and press down firmly with the back of a spoon. Put the tin in the freezer and leave to set for 30 mins.
Put a large piece of foil (extra-wide foil if possible) on the work surface, or criss-cross two pieces of small foil. Place the cake tin in the centre of the foil and bring up the sides to create a foil bowl around the cheesecake. Place in a medium-sized roasting tin.
Heat oven to 180C/160C fan/gas 4. To make the filling, put the eggs and egg yolks in a food processor and blitz until well combined. Add the cheese, cream, sugar, orange zest and orange liqueur to the food processor and pulse until only just combined. Pour the cheesecake filling gently onto the chilled base.
Add enough just-boiled water to the roasting tin to rise roughly 2cm up the sides. Carefully place the roasting tin in the centre of the oven and bake for about 1 hr 15 mins. The cheesecake is ready when it is very lightly browned and only just set.
Take out of the oven and sprinkle with the grated chocolate. Leave to cool in the water bath for 15 mins. Lift the cake tin from the water and leave to cool for a further 1 hr. Cover with cling film and chill the cheesecake overnight before serving.
To serve, run a round-bladed knife around the cheesecake and carefully release it from the tin. Using a palette knife, slide the cheesecake onto a flat serving plate or cake stand and cut into wedges to serve.