Pour the milk into a sauce pot and heat it over medium high heat until it just comes to a simmer. In the meantime, add the egg yolks, cornstarch, granulated sugar, and salt into a bowl. Whisk the ingredients together until it turns into a pale yellow color.
When the milk is simmering, remove it from the heat. Scoop a ladleful of the hot milk then slowly pour it into the egg mixture while whisking quickly to prevent the eggs from scrambling. Continue adding in the milk while whisking until half of the milk has been used.
Pour the mixture back into the pot with the remaining milk and whisk. Return the pot to the stove and heat over medium heat. Whisk constantly until the cream starts bubbling and thickens into a custard. Remove the pot from heat and mix in the vanilla extract and unsalted butter until fully incorporated.
If your pastry cream is lumpy, you can pass it through a fine mesh sieve. Otherwise, transfer the pastry cream into a bowl and cover with plastic wrap. Make sure the plastic wrap makes contact with the surface of the pastry cream to prevent a skin from forming. Place it in the fridge to cool completely.
Add the heavy cream and powdered sugar into the bowl of a stand mixer. Use the whisk attachment and mix on high speed until you get soft peaks. When the pastry cream has cooled, mix it with a rubber spatula to loosen it. Add about 2½ cups of the whipped cream into the pastry cream. Gently fold them together until evenly combined.
Spread a layer of the vanilla wafers into a 9x13 inch pan or 2½ – 3 quart baking dish. It should be just enough to cover the bottom of the dish in a single layer. Add on a layer of the sliced bananas. Drizzle some of the salted miso caramel over the bananas. Sprinkle a touch of flaky salt on top. Then spread on a layer of the pastry cream so that that bananas are fully covered. Drizzle another layer of the caramel on top. Then spread on a layer of whipped cream.
Repeat this process one more time to create another set of layers. Garnish the top with a drizzle of the caramel. Crush any remaining vanilla wafers into pieces and sprinkle them on top. Finish with some flaky salt. For best consistency, let the banana pudding set over night. Enjoy!