2 1/2 cups graham cracker crumbs
1/3 cup sugar
1/4 cup mashed banana
1/4 cup (1/2 stick) unsalted butter, melted
1/2 cup sugar
1/3 cup cornstarch
1/4 teaspoon salt
1 1/2 cups whipping cream
1 1/2 cups whole milk
3 large egg yolks
1/2 vanilla bean, split lengthwise
2 tablespoons (1/4 stick) unsalted butter
1 teaspoon vanilla extract
5 ripe bananas (about 1 1/2 pounds total), peeled, cut crosswise into 1/4-inch-thick slices
Set oven at 350°F. Mix graham cracker crumbs, sugar, and mashed banana together for crust. Add unsalted butter, 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 saucepan. 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, at least 8 hours and up to 1 day.