Combine brown sugar, cornstarch, and salt in a 1-quart casserole.
Gradually stir in milk.
Microwave at HIGH 3 to 4 minutes until smooth and thickened, stirring with a wire whisk every 2 minutes.
Beat egg yolk until thick and lemon colored.
Gradually stir about one-fourth of hot mixture into yolk, add to remaining hot mixture, stirring constantly.
Microwave at MEDIUM HIGH (70% power) 1 minute.
Add butter and vanilla, stirring until butter melts.
Arrange bananas in bottom of graham cracker crust.
Pour filling mixture over bananas; cover with waxed paper, and let cool 30 minutes.
Beat whipping cream until foamy; gradually add powdered sugar, beating until soft peaks form.
Spread whipped cream mixture over pie.
Chill at least 3 hours.