Preheat oven to gas 4, 180°C, fan 160°C. Butter a 21cm (8in) round springform tin and line the sides with nonstick baking paper.
Pulse the biscuits, hazelnuts and sugar in a food processor until coarsely ground. Pour in the butter and whizz again until combined. Press the mix evenly into the base of the tin and bake for 10-15 minutes, until golden.
To make the filling, melt the chocolate, either in the microwave or a heatproof bowl over a saucepan of simmering water. Set aside to cool slightly.
Wrap the outside of the tin in two layers of tin foil, scrunching it tightly against the sides to secure it but leaving the top open. Set in a deep roasting tin.
Beat the ricotta and cream cheese with the sugar and cornflour until smooth. Beat in the eggs, little by little, until incorporated. Then beat in the melted chocolate and pour the mix carefully into the tin. Pull the bottom shelf part way out of the oven and put the cheesecake on it. Pour boiling water into the roasting tin, until it reaches halfway up the sides of the cake tin. Slide the roasting tin into the oven and bake for 50 minutes.
When cooked, the cheesecake will be firm but wobble in the middle if gently jiggled. Remove the tin from its water bath and leave to cool for a few minutes on a rack. As soon as it's cool enough to handle, remove the foil then leave to cool completely. Chill for at least four hours, or if you have the time, overnight.
Before serving, drizzle with melted chocolate and scatter with chopped hazelnuts.