PREHEAT oven to 350°F. Reserve 8 pecan halves for garnish. Finely chop 1/2 cup of the remaining pecans. Melt 1/4 cup of the margarine; pour into small bowl. Add 1/4 cup of the finely chopped pecans and the shortbread crumbs; mix well. Press firmly onto bottom and up side of 9-inch pie plate; set aside.
MELT remaining 1/4 cup margarine; pour into medium bowl. Add sugar; beat with electric mixer on low speed until well blended. Add flour, egg, baking powder and vanilla; mix well. Stir in remaining pecan halves. Pour into crust; sprinkle with remaining finely chopped pecans.
BAKE 18 min. or until lightly browned. Serve warm or chilled topped with the whipped topping and the reserved pecan halves.