Line cookie sheet with foil; lightly butter foil. In large heavy skillet, combine sugar and 2 tablespoons butter; cook over medium-high heat for about 5 minutes or until sugar is melted and turns golden brown, stirring constantly. Add 1 cup pecans; cook and stir about 1 minute or until pecans are lightly toasted and well coated with sugar mixture. Quickly spread evenly onto foil-lined cookie sheet. Cool 1 hour or until completely cooled. Break or chop into bite-sized pieces.
Meanwhile, heat oven to 350°F. In medium bowl, combine flour, 1/2 cup pecans and 1/4 cup brown sugar; mix well. With pastry blender or fork, cut in 1/2 cup butter until mixture resembles coarse crumbs. Press in bottom of ungreased 13x9-inch pan. Bake at 350°F. for 10 to 14 minutes or until light golden brown. Cool 30 minutes.
In large bowl, combine pumpkin, 1/2 cup brown sugar and pumpkin pie spice; mix well. Fold in ice cream, 2 cups of the whipped topping and 1 cup of the praline crunch. Spread over cooled crust. Reserve remaining whipped topping and praline crunch for garnish. Cover tightly with foil. Freeze 4 hours or until firm.
To serve, let stand at room temperature for 15 minutes. Cut into squares; place on individual dessert plates. Garnish each square with reserved whipped topping and praline crunch.