Preheat the oven to 350. In a large bowl or stand mixer, combine butter and brown sugar. Mix for 2 minutes until the butter and brown sugar is completely combined.
Add vanilla instant pudding mix, mashed banana, eggs, and vanilla. Mix until combined.
Add flour, cornstarch, baking soda, and salt. Mix until just combined. Add vanilla wafer pieces, white chocolate chips, and mini marshmallows. Mix until just combined.
Scoop the dough out and transfer to a lightly greased or lined cookie sheet. For large, Last Crumb style cookies, use 1/2 cup of dough per cookie. For smaller cookies, use 1/4 cup of dough per cookie.
Bake for 10-12 minutes, until cookies are barely starting to brown. Let the cookies cool and enjoy!
Store any extra cookies in a sealable container for 3-4 days. The cookies will get even softer each day.