For the pudding: In a medium bowl, whisk together the granulated sugar, cornstarch, salt and egg yolks and set aside. In a medium saucepan, heat the milk to just scalding. Remove from the heat and whisk a ladleful of the hot milk into the egg mixture until combined. Add another ladleful of the hot milk to the eggs, whisking until combined, and then add the rest of the milk to the eggs, whisking constantly.
Pour the mixture into a clean saucepan. Set the saucepan over medium-low heat and cook, whisking, until the mixture thickens, about 5 minutes. Continue cooking, whisking constantly, until it is quite thick and glossy, about 10 minutes longer. Remove from the heat and whisk in the butter and vanilla bean paste until completely incorporated. Transfer the mixture to a clean bowl and press a piece of plastic wrap directly onto the surface of the pudding, taking care to cover it completely to prevent a skin from forming. Refrigerate for at least 4 hours.
For the caramelized bananas: Combine the brown sugar, butter and cinnamon in a small saucepan and cook until bubbly. Add the bananas and toss with the mixture to coat. Turn off the heat and stir in the rum.
For the spiked whipped cream: Put the cream, granulated sugar and rum in the bowl of a stand mixer fitted with a whisk attachment and beat on medium speed until soft and billowy, 3 to 5 minutes.
To assemble: Place about 1/4 cup (depending on the size of your serving dishes) of the crushed vanilla wafers in the bottom of each parfait glass. Spoon a few tablespoons of pudding over the crushed wafers. Add a few tablespoons of caramelized bananas on top of the pudding, making sure to include some of the sauce along with the bananas. Top with a dollop of the spiked whipped cream and a whole vanilla wafer.