Crust: Cream butter and sugar in processor until smooth. Add flour, almonds, baking powder and salt and process just until combined. Gather dough into a ball, flatten into a disk and refrigerate 1 hour in plastic wrap. (Can be made 2 days ahead. Keep chilled. Let stand briefly at room temperature to soften before continuing.) Press dough into 9" glass pie plate, trimming excess even with edge of dish. Crimp edge decoratively. Refrigerate 1 hour.
Preheat oven to 350x. Line crust with foil. Fill with dried beans or pie weights. Bake 12 minutes. Remove beans and foil and bake until crust is set and edges are golden brown, about 20 minutes. Transfer to rack and cool.
Filling: Pour water into small bowl. Sprinkle gelatin over. Let stand 10 minutes to soften. Pour milk into medium saucepan. Scrape seed from vanilla bean into milk; add bean. Bring to simmer. Whisk yolks, sugar, and cornstarch in medium bowl until thick. Gradually whisk in hot milk. Return mixture to saucepan and cook over medium heat until mixture just begins to boil and thicken, whisking constantly, about 3 minutes. Add softened gelatin and stir to dissolve. Strain mixture into medium bowl. Press plastic wrap onto surface of pastry cream and chill until cool, about 30 minutes.
Using electric mixer, beat cream and powdered sugar in a large bowl to stiff peaks. Carefully fold 1 1/2 c whipped cream into pastry cream. Fold in bananas. Spoon mixture into crust. Spoon remaining whipped cream into pastry bag with medium star tip. Pipe whipped cream over filling, covering completely. Chill until set, at least 3 hours and up to 6 hours.
If you use the recipe, let me know how it turns out!