In heavy saucepan, dissolve cornstarch in water; stir in sweetened condensed milk and egg yolks. Cook and stir until thickened and bubbly. Remove from heat; add margarine and vanilla. Cool slightly. Arrange 2 bananas on bottom of prepared pastry shell. Pour filling over bananas; cover. Chill 4 hours or until set. Spread top with whipped cream; garnish with remaining banana slices. Refrigerate leftovers.