For crust: Preheat oven to 350°F. Stir graham cracker crumbs, sugar, and mashed banana in large bowl to blend. Add unsalted 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. For filling: 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; cool completely, whisking occasionally, 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.