Prepare the Cookie Dough:
Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
In a mixing bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and mashed banana.
In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Make the Crumble Topping:
In a small bowl, mix the crushed vanilla wafers, flour, melted butter, and brown sugar until crumbly.
Bake the Cookies:
Scoop tablespoon-sized balls of cookie dough onto the prepared baking sheets, spacing them about 2 inches apart.
Gently press some of the crumble topping onto each cookie.
Bake for 10-12 minutes, or until the edges are lightly golden. Allow the cookies to cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.
Prepare the Banana Pudding Drizzle:
In a bowl, whisk together the instant banana pudding mix and cold milk until thickened. Transfer the pudding to a piping bag or a plastic bag with a small corner cut off.
Decorate the Cookies:
Drizzle the banana pudding over the cooled cookies. Allow the drizzle to set before serving.