Preheat your oven to 350°F (175°C).

In a bowl, whisk together flour, baking soda, cinnamon, and salt.

In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until fluffy.

Beat in the eggs one at a time, then add the vanilla extract.

Gradually mix in the dry ingredients until combined.

Stir in the oats until evenly distributed.

Drop tablespoon-sized balls of dough onto a baking sheet lined with parchment paper.

Bake for 12-15 minutes or until edges are golden brown.

Let cookies cool completely on a wire rack.

Mix marshmallow fluff and powdered sugar until smooth. Spread on half of the cookies and top with the remaining cookies.