In heavy pan dissolve cornstarch in water. Stir in condensed milk and egg yolks. Stir and cook until thick and bubbly. Remove from heat and add butter and vanilla. Cool slightly.
Slice 2 bananas and dip in lemon juice; drain and arrange on bottom of crust. Pour filling over this and cover. Chill 4 hours or until set. Spread with whipped cream. Slice remaining banana; dip in lemon and drain. Garnish top of pie.