Early in day: In saucepan, mix cold water with egg yolks; stir in combined sugars, flour, and salt. Gradually add boiling water, stirring briskly. Cook, stirring, 3 to 5 min., or until smooth and thick. Add butter and vanilla; cool 5 min., stirring occasionally. Pour into baked pie shell. Refrigerate till serving time. To serve: Slice bananas over filling. Whip cream; spread over all.