To make the pudding layer, add the cold milk and instant vanilla pudding mix to a large mixing bowl. Using a hand or stand mixer fitted with the whisk attachment, mix on low speed for 2 minutes until the pudding begins to thicken.
Add the sweetened condensed milk and mix again on low speed until smooth and well combined. Set aside to continue thickening while you prepare the whipped cream.
In a medium mixing bowl, combine the heavy cream, powdered sugar, and vanilla extract. Using a hand or stand mixer fitted with the whisk attachment, whip on medium speed until stiff peaks form. Be careful not to overwhip the cream (see note 3).
Using a spatula, gently fold the whipped cream into the pudding mixture until fully combined and no streaks remain. Fold gently to keep the mixture light and fluffy.
Slice the bananas into roughly ⅓-inch thick slices. Wait until just before assembling the pudding to slice them, as they will begin to brown if left sitting for too long.
To assemble, use a standard 8-inch (20 cm) trifle dish. Spread a very thin layer of the pudding mixture (about ¼ cup) over the bottom of the dish. This helps soften the first layer of wafers, which can otherwise remain slightly firm.
Arrange a single layer of vanilla wafers over the pudding, filling in as many gaps as possible. Top with a layer of sliced bananas, leaving a few small gaps between the slices so the banana flavour doesn't become too overpowering. Spread about one-quarter of the remaining pudding mixture evenly over the bananas.
Repeat the layers of vanilla wafers, bananas, and pudding until all of the pudding mixture has been used, finishing with a layer of pudding on top. You should end up with four layers of wafers, bananas, and pudding in total.
Cover the trifle dish with cling wrap and refrigerate for at least 4 hours (see note 4).