Preheat your oven to 350°F (175°C).
In a bowl, mix together flour, baking soda, salt, and cinnamon.
In another bowl, cream together softened butter, brown sugar, and granulated sugar until fluffy.
Beat in the egg and vanilla extract until well combined.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Fold in the rolled oats until evenly distributed.
Drop tablespoons of dough onto a baking sheet lined with parchment paper.
Bake for 10-12 minutes, or until edges are lightly golden.
Allow cookies to cool on the baking sheet for a few minutes before transferring to a wire rack.
Once cooled, spread marshmallow fluff on the bottom of one cookie and sandwich with another.