In a large saucepan, stir together sugar, cornstarch, salt and milk over medium-high heat until bubbly. Reduce heat; cook and stir 2 minutes longer. Remove from heat. Whisk a small amount of hot filling into eggs; return all to pan. Bring to a gentle boil; cook and stir 2 minutes longer.

Remove from heat. Stir in butter and vanilla. Transfer to a bowl and press plastic wrap onto surface of custard to stop a film from forming; refrigerate, covered, until cool.

Spread half of the custard into the graham cracker crust. Top with sliced bananas. Spread remaining custard over the bananas, smoothing out the top.

Beat the heavy whipping cream and powdered sugar until soft peaks form. Spread over the pie. Store in fridge until ready to serve.