2 tablespoons granulated sugar
4 teaspoons cornstarch
3 large egg yolks
1 cup whole milk
10 ounces 70% bittersweet chocolate, finely chopped
1 teaspoon vanilla extract
1 (9-ounce) box chocolate wafers, ground (see note)
8 tablespoons (4 ounces) unsalted butter, melted
4 bananas, sliced, divided
2 cups heavy cream, well chilled
Whisk sugar with cornstarch in medium bowl. Whisk in yolks until combined. Heat milk in medium saucepan over medium heat to scalding. Whisk warm milk into egg mixture and return to pan. Cook over low heat, stirring, until mixture is thickened and just begins to boil. Remove from heat and stir in chocolate until melted. Stir in vanilla and chill until cold, about an hour.
While chocolate mixture is cooling, make the crust. Stir chocolate crumbs with butter until moistened. Firmly press into bottom and sides of pie plate. Scatter 2 bananas over crust.
Whip cream on high speed to soft peak. Fold into chilled chocolate mixture and spoon into pie shell. Chill until set, about 2 hours. Slice remaining bananas to garnish pie and serve.