Preheat the oven to 160°C, (140°C for fan ovens) Gas Mark 3 and set the shelf just below centre.
Lightly grease and base line the cake tin. Wrap the outside of the tin in a double layer of foil (to make a water-tight seal around the base).
Make the base: Blend the biscuits in a food processor or mixing bowl, to fine crumbs and pulse/mix in the butter. Lightly press into the tin making an even layer. Chill.
Make the filling: Blend the cream cheese and sugar together in a food processor. Add the eggs, one at a time, with the processor running on low speed. Add the vanilla and crème fraiche. Mix the cornflour into the liquid coffee to make a smooth paste and add this to the egg mixture. Blend until smooth.
Pour the cheesecake filling onto the biscuit base. Stand the wrapped tin in a deep roasting tin. Pour hot water to come halfway up the sides of the cheesecake tin. Bake in the centre of the oven for 30 minutes.
Turn the oven off, partially open the door and leave the cheesecake in the oven for 15 minutes to set. This helps to avoid the top cracking. Remove to a rack to finish cooling and then refrigerate for at least 4 hours before serving.
Make the ganache topping: Heat the cream with the sugar until almost boiling. Pour over the chocolate and stir until melted. Leave to cool and thicken slightly. Pour onto the centre of the top of the cheesecake and tilt to spread evenly. Allow to set.