In a large bowl, mix the butter and brown sugar until well combined.
3. Step
Add the molasses, vanilla, and eggs, and whisk thoroughly.
4. Step
Sift in the flour, baking soda, salt, and cinnamon. Mix until just combined.
5. Step
Add the oats and mix until incorporated. Be careful not to overmix the batter or it might result in tougher cookies.
6. Step
Using an ice cream scoop (or about 2 tablespoons),scoop out cookies onto a baking sheet lined with parchment paper, leaving about 3 inches (8 cm) between each one.
7. Step
Bake for 7-9 minutes. The cookies will still be very soft, so be sure to let them cool thoroughly.
8. Step
While cooling, make the filling by beating the butter, powdered sugar, cream, vanilla, and marshmallow fluff until well combined.
9. Step
Once the cookies are cooled, put a spoonful of the filling on one cookie and sandwich with another. Repeat until all the sandwiches are made.
