For the Pudding

1. In a medium saucepan, stir sugar, cornstarch, and salt together. In a separate bowl, combine egg yolks and milk and whisk together. Over medium heat, gradually whisk the milk and egg mixture into the cornstarch mixture. Cook on medium heat stirring constantly until thickened and boiling. Boil for one minute then remove the pan from the heat. Stir in softened butter and vanilla.
2. Cool the pudding to room temperature. An ice bath will speed this up if needed.

For the Whipped Cream

1. Add the heavy whipping cream, sugar and vanilla to the bowl of an electric mixer with the whisk attachment fitted. Turn mixer to medium speed for a minute, then increase speed to high and the cream thickens. Whisk until cream has thickened to soft peaks, about 2 more minutes.

For Assembly

1. Arrange eight 1-cup jars on a tray or baking sheet.
2. Gently fold 1/3 of the whipped cream into the cooled pudding, being careful not to stir the air out of the whipped cream. Pour the pudding into a gallon size Ziploc bag, squeeze the air out, and seal the bag. Cut a 1/2 inch hole in one of the corners. Pipe a small dollop of pudding into the bottom of each jar. Press three whole vanilla wafer cookies into each jar in a single layer. Pipe another layer of pudding to cover the cookies, then three slices of banana. Add a tablespoon or so of crushed cookies over the bananas, reserving a 1/4 cup for garnish on the very top. Pipe the remaining pudding over the crushed cookie layer, and then top with a spoonful of the remaining whipped cream. Sprinkle with crushed cookies. Cover the cups with a sheet of plastic wrap and refrigerate for 3-4 hours, so that the cookies have time to soften. Top with a banana slice or two just before serving if desired.
3. These pudding cups are best served anywhere from 3-24 hours after assembling.