a chinois or fine-mesh sieve; a 9-inch tart pan with a removable bottom; a kitchen blow torch
For the tart crust: Pulse the walnuts and sugar in a food processor until they are finely ground, about 6 pulses. Add the cold butter and pulse until the walnuts and sugar come together with the butter, but the mixture is not homogenous. Add the flour and salt and pulse again until the butter is the size of small peas. Turn the dough out onto a work surface and knead until it just comes together. Form into a disk, wrap in plastic wrap and refrigerate until firm, 30 minutes to 1 hour.
Lightly flour the dough and roll it between 2 pieces of parchment or plastic wrap to about 1/4 inch thick. Press into the bottom and up the sides of a 9-inch tart pan with a removable bottom, patching any holes. Use a fork to gently prick the crust, ensuring that steam can escape while it bakes. Freeze until hard, 6 to 8 hours.
Preheat the oven to 375 degrees F. Bake the tart crust until pale golden, 15 to 20 minutes. Cool completely on a wire rack.
For the pastry cream: While the tart crust dough is chilling, bring the milk, vanilla bean paste and salt to just under a boil in a medium pot over medium heat. Meanwhile, whisk the sugar, cornstarch and egg yolks in a medium bowl until smooth and combined. Whisking constantly, ladle half the hot milk into the egg yolk mixture, one ladle at a time. Continue until all the hot milk has been whisked in and the egg mixture is now warm.
Return the milk and egg yolks to the pot and cook over medium-low heat, whisking, until thickened to a pudding consistency, 3 to 5 minutes. Immediately strain through a chinois or fine-mesh sieve into a large airtight container. Stir the butter cubes into the pastry cream until melted and combined. Cover with plastic wrap, pressing it directly onto the surface of the pastry cream, and refrigerate until chilled through, about 2 hours.
For the assembly: Spread the fudge sauce over the cooled tart shell and top with an even layer of thinly sliced bananas (using about half). Add a layer of pastry cream (about 1 cup) and smooth into an even layer; reserve the remaining cream for another use (see Cook's Note). Top with the remaining sliced bananas, shingling them slightly and arranging them in a concentric circle. Sprinkle the sugar over the bananas and brulee with a kitchen blow torch until golden brown. Garnish with some chopped walnuts.
The remaining pastry cream will keep in an airtight container for up to 5 days in the refrigerator.
Propane gas torches are highly flammable and should be kept away from heat, open flame and prolonged exposure to sunlight. They should be used only in well-ventilated areas. Follow torch manufacturer's instructions for use.