In a medium saucepan, whisk together the milk, sugar, cornstarch, egg, and egg yolk.
Once well-combined, place the pan over medium-heat.
Whisk often to prevent lumps from forming.
Making this custard takes a bit of patience. You are stirring and stirring and then all of a sudden it starts getting thick. So that's why you need to whisk constantly.
Cook until thick and bubbly.
Remove the pan from the heat and add the butter and banana extract (or vanilla extract) and whisk until completely melted.
In an 8-inch (2 quart) baking dish, place a layer of bananas, then top that with a layer of vanilla wafers.
Pour 1/2 of the custard over the wafers and spread to the edges.
Repeat this process once more.
Cover with plastic wrap and chill for at least four hours.
For the whipped topping:
Whip the heavy cream with the vanilla and the powdered sugar until it reaches soft peaks. Spread the whipped cream over the chilled pudding. Top with crushed vanilla wafer cookies.