Whisk sugar, cornstarch and salt in a saucepan; whisk in milk. Cook over medium heat, stirring, until bubbling and thick.
Whisk yolks in a bowl. Slowly whisk in 1 cup hot milk mixture; return to saucepan. Bring to a gentle boil, stirring; cook 2 minutes.
Remove from heat; stir in butter and vanilla. Cool 15 minutes, stirring occasionally.
Slice bananas into pie shell. Spread custard over them; cover surface with plastic wrap. Chill at least 4 hours.
Whip cream and powdered sugar to soft peaks; spread over chilled pie. Keep refrigerated.