Preheat oven to 350 degrees F. In a standing mixer fitted with a paddle attachment, cream butter and sugar together. Scrape down sides as needed. Add egg, vanilla, and dry ingredients.
Prepare two sheet trays with parchment paper. Scoop out 1 teaspoon sized balls and place them on the tray, 1-inch apart from each-other. Continue to do this until you are out of dough.
Bake vanilla wafers for about 15 minutes, or until edges around are golden brown.
To make custard, set up a double boiler by filling a pot with 1-inch of water. Turn heat on medium.
In a glass or metal bowl, combine yolks, sugar, milk. Put bowl over steaming pot and whisk until the eggs have thickened. Whisk in the mashed banana, lemon juice, and vanilla bean seeds. Set aside.
Whip up cream to medium-stiff peaks. Fold cream into banana mixture.
Now, assemble your pudding.
Line the bottom of the serving dish with vanilla wafers. Add some sliced bananas on top, then a thin layer of banana custard. Repeat until you are flush with the top of your dish.
To top, mix together cinnamon and powdered sugar. Dust the top with cinnamon sugar.
Chill pudding for 24 hours before serving.