1.To prepare the crust: Lightly dust a 9-inch-diameter glass pie dish with cocoa powder or confectioners' sugar. Melt the butter and chocolate together in a heavy small saucepan over very low heat. Pulse the cookies and sugar in a food processor until the mixture resembles coarse sand. Pulse in the melted chocolate-butter mixture until evenly moistened. Press the crumb mixture evenly into the bottom and up the sides of the dish. Freeze until firm, about 30 minutes.
2.Peel and halve each banana, slice each in half lengthwise. Arrange the bananas cut-side down in concentric circles, on the crust.
3.To prepare the custard: Whisk the sugar, cornstarch, and salt together in a medium saucepan. Vigorously whisk in the yolks, and both milks until smooth. Add the vanilla seeds and pod to the pot. Cook over medium heat, stirring constantly, until mixture starts to thicken and simmer, and reaches about 160 degrees F, about 4 minutes. Once the mixture is simmering, continue to cook 1 minute longer, stirring constantly. Remove from heat, remove the vanilla pod and pour or spread the custard evenly over the bananas. Chill for 2 hours and up to overnight.
4. To prepare the topping: Beat the heavy cream with an electric mixer until it just starts to thicken. Beat in the confectioners' sugar and continue beating until just stiff peaks form. Gently fold in the sour cream, then spoon onto pie to cover the custard. Serve immediately or chill for up to 4 hours.
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