Bake 1/2 cup hazelnuts in a shallow pan at 350°, stirring occasionally, 15 minutes or until toasted. (The skins will be very dark.) Wrap nuts in a clean kitchen towel; let stand 20 minutes. Rub nuts briskly to loosen skins; discard skins. Chop nuts; set aside.
Bring prunes and water to cover to a boil in a medium-size nonaluminum saucepan over high heat; reduce heat, and simmer 30 minutes. Cool; drain.
Process sugar and next 3 ingredients in a food processor until blended, stopping to scrape down sides.
Spread prunes in Pastry Tart Shell; sprinkle with hazelnuts. Pour crème fraîche mixture over nuts. Place tart in a larger shallow pan.
Bake at 375° for 55 minutes or until set, shielding with aluminum foil after 40 minutes to prevent excessive browning. Cool on a wire rack. Garnish with sweetened whipped cream and chopped toasted hazelnuts.