Make the pastry using the rubbing-in method. Chill in the fridge for 15 minutes, then use ½ to line the 25cm fluted flan tin. To make the filling, place the ricotta cheese in a sieve for 10 minutes to drain off any excess water. Place the ricotta in a bowl, along with the caster sugar, eggs, lemon zest, chopped hazelnuts, vanilla essence and chopped chocolate. Mix together. Next, spoon the chopped pears into the base of the prepared pastry case and even out. Carefully spoon over the ricotta filling and spread out using a palette knife. Use the remaining pastry to cut out thin strips and arrange them on top of the ricotta to create a lattice design. Place the ricotta tart in the oven at 180`c, and bake for 40 minutes or until the pastry is golden brown. Remove from the oven and allow to cool down completely. Dust the top of the ricotta tart with icing sugar and serve.