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. Chill 4 hours or until set.
Spread top with whipped cream. Slice remaining banana; dip in lemon juice, drain and garnish top of pie. Refrigerate leftovers.
Serving Size: 6