Start by simmering a banana and milk over low heat for 10 minutes. Remove from heat and let it come to room temp before straining the bananas from the milk. In a medium bowl, whisk the egg yolks, cornstarch, and ½ cup of milk set aside.
Add the banana infused milk back to the saucepan along with ½ cup of sugar and bring it to a boil over low heat. While whisking, slowly ladle ⅓ of the hot milk into the egg mixture to temper the eggs.
Begin to slowly pour the egg mixture back into the pot while whisking constantly. Let it cook for 1-2 minutes to thicken then remove it from the heat. Immediately pour the mixture into a bowl over an ice bath.
Place plastic wrap directly on the pudding top to avoid a skin from forming. Cool the pudding to room temperature. Completely chill in the fridge for 2-3 hours before assembling.
Make the whipped cream by beating the heavy cream and powdered sugar to stiff peaks in the bowl using a mixer. Scoop half of the whipped cream into the pudding and fold until light and fluffy. Refrigerate until needed.
Preheat the oven to 325 degrees. Crush 40 vanilla wafers with the brown sugar, 2 tablespoons of granulated sugar, and 4 tablespoons of melted butter. Spread on a baking sheet and toast for 15-18 minutes stirring and roaring the pan after 8-9 minutes. Remove from the oven and set aside to cool.
Layer the bottom of the baking dish with cookies and banana slices. Spread a layer of pudding then repeat with cookies and banana slices and another layer of pudding.
Top with the remaining whipped cream and the vanilla cookie streusel. Chill covered with plastic wrap for at least 6 hours.