Preheat the oven to 375 degrees F.

In the bowl of a food processor, add the graham cracker crumbs, butter, brown sugar and mashed banana. Pulse to combine. Scrape the mixture into a 9-inch springform pan, and spread evenly with a spatula and bake until set and golden brown, 10 to 12 minutes. Set aside on a wire rack to cool completely.

In a medium saucepot, combine the milk and 1/2 cup of the sugar. Halve the vanilla bean lengthwise, scrape out the seeds with the tip of a small knife, and add to the pot along with the pod. Bring to a gentle boil over medium heat, stirring to dissolve the sugar. Remove from the heat.

In a large bowl, whisk the egg yolks until frothy. Add the cornstarch and whisk to dissolve. Slowly add 1/2 cup of the hot milk mixture to the egg yolks, whisking constantly. In a slow, steady stream, continue to whisk in the remaining hot milk mixture. Strain this mixture through a fine-mesh strainer back into your saucepot. Cook over medium heat, stirring constantly with a wooden spoon or rubber spatula until the mixture is thick like pudding, about 8 to 10 minutes.

Transfer the pastry cream to a bowl and cover with plastic wrap, pressing down against the surface to prevent a skin from forming. Refrigerate until well chilled, 3 to 4 hours up to overnight.

In a medium bowl, beat 2 cups of the heavy cream until thick. Add 1/2 cup of the remaining sugar and continue to beat until soft peaks form. Remove and discard the vanilla bean pod from the pastry cream and whisk until smooth. Fold the pastry cream into the whipped cream.

Add the water to a small metal bowl.  Sprinkle the gelatin over the water and allow it to soften for 1 minute. Place the bowl over a small saucepan of barely simmering water and stir until dissolved and no more granules remain, about 3 minutes.  Stir the gelatin mixture into the pastry cream.

Peel and cut the bananas into ¼-inch thick slices.

To assemble the cake, spoon 2/3 cup of the custard over the crust and spread it into an even layer with an offset spatula. Arrange one-third of the banana slices in a tight, tiled pattern over the custard, pressing slightly to push into place. Spread a layer of the custard over the bananas and top with a layer of closely arranged vanilla wafers, about 2 cups, to cover. Spread another layer of custard over the wafers and top with another third of the bananas, followed by another layer of custard and the last portion of bananas. Spread the remaining custard over the top, wrap tightly with plastic wrap, and chill at least 8 hours or overnight.

In a medium bowl, beat the remaining 2 cups cream until thick. Add the remaining 1/2 cup of the sugar and continue to beat until soft peaks. Transfer to a pastry bag fitted with a star tip.

To serve, remove the sides from the springform pan. Slice the cake into 10 pieces. Cover the back edge of each piece with vanilla wafers or soft ladyfindgers and decoratively pipe sweetened whipped cream over the top. Garnish with dried banana chips and drizzle with fudge sauce. Garnish each plate with a mint sprig and serve immediately.