Combine cornstarch, sugar and salt in top of double boiler. Gradually whisk in milk. Cook over simmering water, whisking constantly, until mixture thickens, about 10 minutes. Mixture must boil to thicken.
Lightly beat egg yolks in small bowl. Slowly add about 1/4 cup hot mixture to egg yolks and mix well. Stir egg yolks back into milk mixture in double boiler and cook 5 minutes. Let cool slightly, then stir in butter
and vanilla until butter melts. Add banana slices. Pour into cooled baked pie shell and chill until set, about 1-1/2 hours. Top with sweetened whipped creawm.