Put the flour, salt and sugar in the bowl of a food processor and pulse a few times to combine. Sprinkle half of the butter cubes over the top of the flour and process for 10 seconds. Sprinkle the remaining half of the butter over the mixture and pulse until the mixture resembles coarse cornmeal, with some butter bits no bigger than peas. With the motor running, add the buttermilk through the feed tube and process until the dough just begins to come together. Transfer to a flat surface and lightly knead until the dough comes together. Form into 2 flat disks, wrap in plastic wrap and refrigerate for at least 1 hour. (You will need only 1 for the pie; the remaining dough can be frozen for up to 1 month.)
Lightly spray a 9-inch tart pan with a removable bottom with nonstick spray. Roll the dough on a lightly floured surface into a 12-inch round that is 1/8-inch thick and fit it into the tart pan. Roll rolling pin over tops of pan to trim dough flush with rims and lightly prick bottoms and sides all over with a fork. Place in the freezer for 30 minutes.
Preheat the oven to 375 degrees F. Place the pan on a baking sheet. Spray the shiny side of a 12 piece of aluminum foil and line the shell shiny-side down with the foil and fill with pie weights. Bake on the middle rack in the oven until the crust sets and turns a pale golden brown, about 20 to 25 minutes. Remove the foil and pie weights and continue baking until golden brown and baked through, about 15 minutes longer. Transfer the tart pan to a baking rack and let cool completely, about 1 hour.
Melt the butter in a large saute pan over medium-high heat, add 1 1/2 tablespoons of brown sugar and cook, stirring constantly, until the sugar is melted and the mixture is caramelized, about 4 minutes. Add the bananas and cook until bananas start to break down, and caramelize on both sides. Add 2 cups of the milk, bring to simmer and scrape the bottom and sides of pan and carefully transfer the mixture to a medium saucepan. Puree the mixture with an immersion blender until smooth; bring to a simmer.
In a bowl whisk the sugar, cornstarch and salt together until combined. Add the remaining 1 cup of milk and whisk until the cornstarch is dissolved. Whisk this mixture into the banana mixture and cook, whisking constantly, until the mixture thickens, about 4 minutes.Whisk together the remaining tablespoon of brown sugar and yolks in a small bowl until smooth. Ladle some of the thickened mixture into the egg mixture to temper the yolks. Pour the yolk mixture into the pot and cook until thickened, 3 to 5 minutes, WHISKING THE ENTIRE TIME. Once the mixture has thickened let it come to a boil for exactly 1 minute, whisking a few times. Remove from the heat, whisk in the softened gelatin and vanilla and strain through a coarse mesh strainer into a bowl.
Cover the surface of the custard with plastic wrap and chill in the refrigerator until set, at least 4 hours and up to 12 hours. Once pudding is set, whip the custard in the bowl of a stand mixer with a whisk attachment on high speed until smooth, about 20 seconds. Add the heavy cream and whip until light and fluffy, about 20 seconds longer.
Transfer the filling to the cooled pie shell. Place plastic on top and let chill in refrigerator for at least 2 hours. Mound the top with the hazelnut whipped cream and sprinkle with some of the hazelnut brittle just before serving.
Preheat oven to 325 degrees F. Spread hazelnuts on a baking sheet and bake until lightly toasted, about 5 minutes. Let cool slightly and coarsely chop.
Combine sugar and water in a medium saucepan and cook (without stirring) over high heat until it becomes a deep amber caramel, about 8 minutes. Remove from the heat and stir in the hazelnuts. Transfer to a baking sheet lined with a silpat. Cover with another silpat and using a rolling pin, roll the mixture as thin as possible. Let sit until the caramel has hardened, about 20 minutes. Once hard, removed from the silpan and chop into small pieces, using a chef's knife.
Combine cream, vanilla seeds, sugar and Frangelico in a bowl and whip using a balloon whisk or hand held mixer until soft peaks form.