Preheat the oven to 400°. Stem, peel and halve the pears; using a melon baller, scoop out the cores. In a medium bowl, toss the pears with the sugar, lemon juice, vanilla and 1 tbsp. water. In a 2-qt. baking dish, arrange the pears in a single layer, cut side down. pour the juices from the bowl over the pears. Bake until tender, about 45 minutes. Let the pears cool, basting occasionally with the juices in the dish.
On a lightly floured surface, roll out the pastry to a 12-inch square, then trim to an 11-inch round. Fit the pastry into a 9-inch pie pan; crimp the edges. Refrigerate for 15 minutes. Line the crust with foil and fill with pie weights or dried beans; bake for 22 minutes. Remove the foil and weights and continue baking until golden, about 8 minutes. Let cool on a rack.
Spread the chocolate-hazelnut spread on the bottom and up the sides of the crust. Slice the pear halves lengthwise into eighths and arrange in the piecrust. Sprinkle with the hazelnuts. Serve within 4 hours, with dollops of creme fraiche, if using.