MAKE THE BANANA CREAM. In a large bowl, whisk the instant pudding mix and the milk for 2 minutes; let sit until thickened, about 5 minutes.
Add the mashed banana and stir until well combined; transfer to a piping bag and set aside.
MAKE THE SHAKES. In a high-powered blender, blend the ice cream and milk until thick and smooth in consistency, adding more milk, as needed.
To ASSEMBLE, pipe half of the banana cream filling evenly into six (12-ounce) milkshake glasses.
Sprinkle half of the vanilla wafer crumbles evenly over the cream.
Pour half of the vanilla shake mixture into the glasses.
Repeat with the remaining ingredients.
Top with whipped cream and garnish with additional vanilla wafer crumbs and a whole vanilla wafer; serve immediately.