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 butter and vanilla. Cool slightly. Slice 2 bananas; dip in lemon juice and drain. Arrange on bottom of prepared crust. Pour filling over bananas; cover and chill 4 hours or until set. Spread top with whipped cream. Slice remaining bananas; dip in Realemon; drain and garnish top of pie drizzle caramel on top if wanted. Refrigerate leftovers.