Pie Crust: In a bowl, stir together flour, sugar and salt. Using a pastry
blender, cut butter cubes into flour mixture until pieces are pea-size. Sprinkle
vinegar over flour mixture, tossing with a fork. Sprinkle 5 to 6 tablespoons
cold water, 1 tablespoon at a time, over flour mixture, tossing with a fork
each time until dough is moistened and crumbly. Press crumbs firmly on bottom
and up sides of a 9-inch pie plate.
Filling: Heat oven to 375 degrees F.
In a bowl, stir together pumpkin, brown sugar, pumpkin pie spice, flour,
eggs, Frangelico and sour cream; mix well. Pour into pie crust, fold narrow
strip of foil over edges of crust to prevent overbrowning; bake 30 minutes.
Meanwhile, make topping. In a bowl, combine flour, brown sugar and Frangelico.
Cut in butter until crumbly. Stir in hazelnuts.
Remove foil from pie and sprinkle with topping.
Bake 15 to 20 minutes more or until knife inserted near center comes out