Preheat oven to 350°F. Stir graham cracker crumbs, sugar, and mashed banana in large bowl to blend. Add butter and stir to moisten evenly. Press onto bottom and up sides of 10-inch-diameter glass pie dish. Chill until firm, about 30 minutes.
Bake crust until set and pale golden, about 15 minutes. Cool completely.
Whisk sugar, cornstarch, and salt in heavy medium saucepan to blend. Gradually whisk in whipping cream and whole milk, then egg yolks. Scrape in seeds from vanilla bean; add vanilla bean. Whisk over medium-high heat until custard thickens and boils, about 6 minutes. Remove from heat. Whisk in unsalted butter and vanilla extract. Discard vanilla bean. Transfer custard to large bowl and cool completely, whisking occasionally, for about 1 hour.
Stir custard to loosen, if necessary. Spread 1 cup custard over bottom of prepared crust. Top with half of sliced bananas, then 1 cup custard, covering bananas completely. Repeat layering with remaining bananas and remaining custard. Chill banana cream pie until filling is set and crust softens slightly, at least 8 hours and up to 1 day. Cut pie into wedges and serve.