Line large cookie sheet with parchment paper and preheat oven to 350 degrees.
Combine vegetable oil, eggs, cake mix, water, and pudding mix in a large mixing bowl. Stir until blended and then mix for about two minutes. Stop when texture is creamy.
Scoop two tablespoons onto parchment paper for each cookie. Leave two inches between each cookie.
Bake for ten to thirteen minutes. Remove and cool completely on wire racks.
Filling
Mix softened butter until creamy.
Add in powdered sugar and vanilla extract and mix until texture is smooth.
Hand stir in your marshmallow fluff until evenly blended.
Put in refrigerator for about ten minutes.
Putting It All Together
Place cooled cookies flat side down.
Using a spoon or piping bag add cream to half of your cookies.