Start by making the pudding. In a large saucepan, whisk together the cornstarch and salt. Set aside. In a medium bowl, add the egg yolks and sugar. Mix very well with a fork until light and ribbony, this will take a few minutes. Set aside. To a blender, add the ripe banana and 1 cup of the milk. Blend until smooth.
Add the "banana milk" along with the remaining 2 cups of milk to the saucepan and place over medium heat. Whisk constantly and bring to a boil. Let boil 2 minutes while whisking. In a slow stream pour about 1/3 of the pudding into the egg mixture and whisk very well to temper the eggs.
Slowly pour the egg mixture into the saucepan and whisk constantly while bringing the pudding to a boil once more. Once boiling, remove from heat and whisk in the vanilla and butter. Let the pudding cool completely, whisking occasionally. Once cooled, refrigerate for about 2 hours.
To make the whipped cream, add the heavy cream to a large bowl and freeze for 10 minutes. Remove from freezer and add the vanilla and powdered sugar. Beat on medium-high speed until very stiff peaks form, about 5-10 minutes.
Fold about ¾ of the whipped cream into the chilled pudding. To assemble, place a layer of banana slices into the bottom of the cups, add a layer of Nilla Wafers then a scoop of pudding. Repeat this process once and top with a little remaining whipped cream. Refrigerate for at least 2 hours. When ready to serve, garnish with crushed wafers and more banana slices.