To Make the Custard: In a large (2 quart/2 liter) microwave-safe bowl, whisk together the egg yolks, sugar, cornstarch and salt, then add in the milk and mix until smooth.
Microwave on high for 4 minutes. Whisk briefly, then microwave again for another 2 minutes. Carefully whisk again. At this point you may notice that it has thickened a lot. If it is still quite runny, microwave for 30 seconds at a time until it gets thick and bubbles up.
Give it another whisk and microwave for one more full minute. (Important note: Ours took less than 8 minutes in our 1200 watt microwave. Your timing might vary depending on the power of your microwave.)
Whisk in the butter and vanilla extract, then strain the pudding into a clean, heat-safe bowl and place a piece of plastic wrap directly on the surface of the pudding to prevent skin from forming.
Allow to cool down at room temperature before placing in the fridge for about 3 hours, or until completely cold. (You can make this custard up to 24 hours in advance.)
To Assemble the Pudding: In the bowl of a stand mixer fitted with a whisk attachment (or with a medium bowl and a handheld electric mixer), combine the heavy cream, powdered sugar and vanilla extract. Whip on medium-high speed to medium-stiff peaks (The peaks should hold their shape pretty well, except that the tip of the peak curls over on itself when the beaters are lifted.)
Fold about ¼ of the whipped cream into the chilled custard to lighten it and set the rest aside in the refrigerator.
Spread ⅓ of the custard in the bottom of a serving bowl*. Layer half of the Nilla wafers over the custard followed by half of the banana slices.
Spread another ⅓ of the custard on top of the bananas, then layer on the remaining wafers and bananas.
Top with the rest of the custard, making sure to cover the banana slices completely to prevent browning.
Let chill for at least 2 hours (or up to 4 hours) to allow the pudding to set and the flavors to meld. Just before serving, give the whipped cream a quick whisk by hand to refresh it, then dollop it on top of the pudding.
**This is best the day it is made as the bananas turn brown very quickly.