To make the pudding:
Whisk the eggs with the vanilla in a large bowl and set aside. Bring the milk, heavy cream, sugar, and salt to a simmer in a large saucepan over medium-high heat. Turn off the heat and whisk a little of the hot liquid into the egg mixture. Continue adding more hot milk until the eggs are tempered and the bottom of the bowl is warm to the touch and then return the egg mixture to the saucepan with the remaining milk. Turn the heat to medium-high and cook, stirring constantly with a wooden spoon or whisk, until the pudding thickens and you can draw a clear line through the custard on the back of a spoon, about 5 minutes (don't let the custard boil--this will cause the eggs to curdle). Pour the custard through a medium sieve and into a medium bowl. Whisk to cool slightly, cover flush with plastic wrap, and refrigerate until it is completely cool, about 2 hours.
To make the caramel sauce:
Microwave together the butter, heavy cream, and salt until the butter is melted (or melt the butter with the cream and salt in a small saucepan). Whisk to combine and set aside. Place the sugar and lemon juice in a medium saucepan. Melt the sugar over medium-high heat, swirling the pan occasionally to evenly distribute the heat, until the sugar is a deep nutty brown and smells bittersweet, 10 to 12 minutes. Pour in the butter mixture (be careful-the sauce will hiss and bubble up) and then place the mixture back over medium-high heat to return it to a boil. Turn off the heat and set the saucepan aside to let the caramel cool to room temperature, 1 to 2 hours.
To make the whipped cream:
Place the heavy cream, sugar, and vanilla (if using a vanilla bean, split the pod and scrape out the seeds, adding them to the cream; save the pod and place it in a jar of sugar to infuse it with vanilla flavor) in the bowl of a stand mixer (or large bowl if using a hand mixer) and whip it on medium speed until it is frothy. Increase the speed to medium-high and whip until you get stiff peaks. Refrigerate until you're ready to assemble the pudding.
Peel and thinly slice the bananas and set aside (wait to slice the bananas until you're ready to assemble the dish, otherwise, they'll brown). Evenly spread 1 cup whipped cream over the bottom of a large trifle dish or punch bowl. Top with a layer of bananas and 1 cup/240 ml pudding. Place about 20 wafers in a flat layer on top of the pudding and evenly drizzle 1/2 cup/120 ml caramel over the wafers. Repeat the layering process three times beginning with the whipped cream, followed by some sliced bananas, pudding, wafers, and caramel. Finish the pudding with a final layer of whipped cream (you should have about 1 cup remaining after repeating four layers), cover with plastic wrap, and refrigerate for at least 6 hours or overnight. Serve in dessert bowls.
Reprinted with permission from Masala Farm by Suvir Saran, Chronicle Books copyright (c) 2011