Preheat oven to 375°F.
In bowl, stir together pumpkin, brown sugar, pumpkin pie spice, flour, eggs, liqueur and sour cream; mix well. Pour into piecrust, fold narrow strip of foil over edges of crust to prevent overbrowning; bake 30 minutes.
Meanwhile, make topping. In bowl, combine flour, brown sugar and liqueur. 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 clean. Cool on wire rack.
Published in the November, 2005 issue of Marketplace.