In a medium saucepan, cook potatoes, covered, in 2 cups boiling water about 30 minutes or until tender. Drain. Using a potato masher or an electric mixer, mash or beat potatoes until almost smooth (you should have about 11/2 cups). Add milk and beat until smooth.
Preheat oven to 375 degree F. In a large bowl, combine eggs, sweetened condensed milk, and pumpkin pie spice. Stir in potato mixture. Turn mixture into pastry shell. In a small bowl, stir together hazelnuts, brown sugar, and melted butter. Sprinkle mixture evenly over pie.
Cover edge of pie with foil. Bake for 25 minutes; remove foil. Bake for 15 to 20 minutes more or until a knife inserted near the center comes out clean. Cool on a wire rack for 2 hours before serving. Makes 10 servings.