1. Place the butter and sugar together in a food processor and blend until smooth. Add in the egg yolk and blend again until it is thoroughly mixed.
2. Add in the flour and pulse a few times until the dough is smooth and not sticky.
3. Line the greased 9 inch tart tin with the dough
4. Chill for at least 30 minutes.
For the Filling
1. Beat together the butter with the sugar until light and fluffy.
2. Add in the eggs, ground walnuts, flour and beat until smooth. Stir in the melted chocolate. Spoon the mixture over the pastry and smooth it gently.
3. Peel the pears and cut them in half length-wise.
4. Remove the cores and slice nicely.
5. Place sliced pears over the chocolate filling.
6. Bake in a preheated oven at 170C, for about 45 minutes or until the walnut mixture is firm and the pastry is golden brown.
7. Remove it from the oven, let it cool, and dust with powdered sugar or walnuts.