Place hazelnuts in a single layer on an ungreased baking sheet. Bake at 350° for 15 minutes or until skins begin to split. Place hazelnuts in a colander. Rub hazelnuts briskly with a kitchen towel to remove skins; discard skins. Finely chop hazelnuts.
Combine 1/2 cup chopped hazelnuts, graham cracker crumbs, and butter; stir well. Firmly press mixture evenly in bottom and up sides of a buttered 9" pieplate. Bake at 375° for 7 minutes or until lightly browned. Cool completely on a wire rack.
Spoon softened vanilla ice cream into cooled crust. Cover and freeze until almost firm.
Place sugar in a large heavy skillet. Cook over medium heat, stirring constantly with a wooden spoon, until sugar melts and turns golden (about 20 minutes). Add remaining 1/2 cup chopped hazelnuts to melted sugar, stirring well. Working rapidly, pour mixture into a 15" x 10" x 1" jellyroll pan lined with aluminum foil; spread in a thin layer. Cool completely. Coarsely break hazelnut praline candy into pieces.
Reserve 1 cup praline candy pieces for garnish. Crush remaining candy pieces. Fold crushed candy into softened chocolate ice cream. Spread chocolate ice cream mixture over vanilla ice cream layer. Cover and freeze at least 8 hours or until firm.
Top each serving with reserved praline candy pieces and, if desired, a splash of Frangelico.