1. Make crust: Preheat oven to 350ºF. In a medium bowl, combine all crust ingredients. Press onto bottom and up sides of a 9-inch pie plate. Bake for 10 minutes. Let cool on a wire rack.
2. Make filling: Place sugar, cornstarch and salt in a large saucepan and whisk to combine. Whisk in milk. Place over medium-high heat and bring to a simmer, whisking constantly. Cook, continuing to whisk well, until mixture thickens, 3 to 4 minutes. Place yolks in a medium bowl and beat until just combined. Whisking constantly, slowly drizzle in 1 cup of milk mixture. Slowly add remaining milk mixture, whisking constantly. Whisk in rum extract.
3. Pour mixture back into large saucepan and cook over medium heat, stirring constantly, until it just comes to a boil. Gently boil (reduce heat if it starts to boil vigorously) until custard is thick enough to coat the back of a spoon, about 2 minutes. Pour custard through a fine-mesh sieve into a large bowl and fold in bananas. Pour custard mixture into cooled crust. Cover with plastic wrap, lightly pressing wrap directly onto surface of custard. Let cool, then refrigerate until cold, at least 1 hour.
4. Make topping: In a small, chilled bowl, beat heavy cream with sugar and rum extract until soft peaks form. Remove plastic wrap from top of custard and spoon whipped cream over, leaving a border. Sprinkle with chocolate shavings, if desired. Chill pie until ready to serve.